! Extra }, or forgotten endgroup.
endmulticols ->par if@boxedmulticols egroup
                                                balance@columns return@non...
l.24             end{multicols}

?

pointsinrightmargin
bracketedpoints

usepackage{tikz}
usetikzlibrary{arrows, circuits.ee.IEC,positioning, decorations.markings}
usepackage[american voltages, american currents, siunitx]{circuitikz}
usepackage[labelfont=,textfont={it}]{caption}
captionsetup[figure]{name=Fig.,font=footnotesize}

usepackage{graphicx}
usepackage{multicol}
usepackage{adjustbox}

makeatletter
newenvironment{multichoices}[1][2]{%
begin{multicols}{#1}}{%
    if@correctchoice endgroup fi
end{multicols}}
makeatother

begin{document}
begin{questions}
question
begin{minipage}[t]{0.65linewidth}
Consider the graph shown in the Fig.~ref{fig:Q1}, which of the following is NOT textit{a tree} of this graph?
begin{choices}
    begin{multichoices}
        choice
            adjustbox{valign=t}{%
                scalebox{0.5}{%
                begin{tikzpicture}[anchor=center, x=4cm, y=4cm]
                    coordinate [label=below left:textcolor{blue}{}] (A) at (0.0,0.0);
                    coordinate [label=above left:textcolor{blue}{}] (B) at (0.0,0.8);
                    coordinate [label=above right:textcolor{blue}{}] (C) at (0.8,0.8);
                    coordinate [label=below right:textcolor{blue}{}] (D) at (0.8,0.0);
                    draw (C) to (B);
                    draw (D) to [bend right=55] (1.1,0.9) to (1.1,0.9)
                        to [bend right=55] (B);
                    draw (B) to [bend right=40] (-0.4,0.45) to (-0.4,0.35)
                        to [bend right=40] (A);
                    fill (B) circle(2pt);
                    fill (D) circle(2pt);
                    fill (A) circle(2pt);
                    fill (C) circle(2pt);
                end{tikzpicture}
                } %EOS
            }% adjustbox
        choice
            adjustbox{valign=t}{%
                scalebox{0.5}{%
                begin{tikzpicture}[anchor=center, x=4cm, y=4cm]
                    coordinate [label=below left:textcolor{blue}{}] (A) at (0.0,0.0);
                    coordinate [label=above left:textcolor{blue}{}] (B) at (0.0,0.8);
                    coordinate [label=above right:textcolor{blue}{}] (C) at (0.8,0.8);
                    coordinate [label=below right:textcolor{blue}{}] (D) at (0.8,0.0);
                    draw (A) to (C);
                    draw (D) to [bend right=55] (1.1,0.9) to (1.1,0.9)
                        to [bend right=55] (B);
                    draw (B) to [bend right=40] (-0.4,0.45) to (-0.4,0.35)
                        to [bend right=40] (A);
                    fill (B) circle(2pt);
                    fill (D) circle(2pt);
                    fill (A) circle(2pt);
                    fill (C) circle(2pt);
                end{tikzpicture}
                } %EOS
            }% adjustbox
    end{multichoices}par
    begin{multichoices}
        CorrectChoice
            adjustbox{valign=t}{%
                scalebox{0.5}{%
                begin{tikzpicture}[anchor=center, x=4cm, y=4cm]
                    coordinate [label=below left:textcolor{blue}{}] (A) at (0.0,0.0);
                    coordinate [label=above left:textcolor{blue}{}] (B) at (0.0,0.8);
                    coordinate [label=above right:textcolor{blue}{}] (C) at (0.8,0.8);
                    coordinate [label=below right:textcolor{blue}{}] (D) at (0.8,0.0);
                    draw (A) to (B);
                    draw (C) to (B);
                    draw (D) to [bend right=55] (1.1,0.9) to (1.1,0.9)
                        to [bend right=55] (B);
                    draw (B) to [bend right=40] (-0.4,0.45) to (-0.4,0.35)
                        to [bend right=40] (A);
                    fill (B) circle(2pt);
                    fill (D) circle(2pt);
                    fill (A) circle(2pt);
                    fill (C) circle(2pt);
                end{tikzpicture}
                } %EOS
            }% adjustbox
        choice
            adjustbox{valign=t}{%
                scalebox{0.5}{%
                begin{tikzpicture}[anchor=center, x=4cm, y=4cm]
                    coordinate [label=below left:textcolor{blue}{}] (A) at (0.0,0.0);
                    coordinate [label=above left:textcolor{blue}{}] (B) at (0.0,0.8);
                    coordinate [label=above right:textcolor{blue}{}] (C) at (0.8,0.8);
                    coordinate [label=below right:textcolor{blue}{}] (D) at (0.8,0.0);
                    draw (D) to (C);
                    draw (D) to [bend right=55] (1.1,0.9) to (1.1,0.9)
                        to [bend right=55] (B);
                    draw (B) to [bend right=40] (-0.4,0.45) to (-0.4,0.35)
                        to [bend right=40] (A);
                    fill (B) circle(2pt);
                    fill (D) circle(2pt);
                    fill (A) circle(2pt);
                    fill (C) circle(2pt);
                end{tikzpicture}
                } %EOS
            }% adjustbox
    end{multichoices}
end{choices}
end{minipage}hfill%
begin{adjustbox}{minipage={0.30linewidth},valign=t}
    centering%
    scalebox{0.68}{%
    begin{tikzpicture}[anchor=center, x=4cm, y=4cm]
        coordinate [label=below left:textcolor{blue}{}] (A) at (0.0,0.0);
        coordinate [label=above left:textcolor{blue}{}] (B) at (0.0,0.8);
        coordinate [label=above right:textcolor{blue}{}] (C) at (0.8,0.8);
        coordinate [label=below right:textcolor{blue}{}] (D) at (0.8,0.0);
        draw (A) to (B);
        draw (C) to (B);
        draw (D) to (C);
        draw (A) to (D);
        draw (A) to (C);
        draw (D) to [bend right=55] (1.1,0.9) to (1.1,0.9) to [bend right=55] (B);
        draw (B) to [bend right=40] (-0.4,0.45) to (-0.4,0.35) to [bend right=40] (A);
        fill (B) circle(2pt);
        fill (D) circle(2pt);
        fill (A) circle(2pt);
        fill (C) circle(2pt);
    end{tikzpicture}} %EoS
    captionof{figure}{label{fig:Q1}}
end{adjustbox}


question
begin{minipage}[t]{0.65linewidth}
    Consider the network graph shown in the figure, which of the following is NOT a textit{tree} of this graph?
    begin{choices}
        begin{multichoices}
            choice
                adjustbox{valign=t}{includegraphics[scale=0.2]{example-image-a}}
            choice
                adjustbox{valign=t}{includegraphics[scale=0.2]{example-image-b}}
        end{multichoices}par
        begin{multichoices}
            choice
                adjustbox{valign=t}{includegraphics[scale=0.2]{example-image-c}}
            CorrectChoice
                adjustbox{valign=t}{includegraphics[scale=0.2]{example-image}}
        end{multichoices}
    end{choices}
end{minipage}hfill%
begin{adjustbox}{minipage={0.30linewidth},valign=t}
    centering%
    scalebox{0.68}{%
    begin{tikzpicture}[x=3.22cm,y=3.22cm]
    coordinate [label=above left:textcolor{blue}{}] (O) at (1.0,1.0);
    coordinate [label=above right:textcolor{blue}{}] (A) at (0.0,1.0);
    coordinate [label=below:textcolor{blue}{}] (B) at (1.0,2.0);
    coordinate [label=below:textcolor{blue}{}] (C) at (2.0,1.0);
    coordinate [label=above:textcolor{blue}{}] (D) at (1.0,0.0);
    draw ($(D)+(0.0,0.0)$) node[circ]{}  to [R, l=5<ohm>] ($(O)+(0.0,0.0)$) node[circ]{}
        to ($(O)+(0.2,0.0)$) to [V, l_=2<volt>] (A) node[circ]{}
        to [R, l=3<ohm>, label/align=straight] (B)
        to [R, l=4<ohm>, label/align=straight] ($(C)+(0.0,0.0)$) node[circ]{}
        to [R, l=2<ohm>, label/align=straight] ($(D)+(0.0,0.0)$)
        to [I, l=2<ampere>, label/align=straight]($(A)+(0.0,0.0)$) node[circ]{} ;
    draw ($(B)+(0.0,0.0)$) node[circ]{} to ($(B)+(0.0,-0.2)$) to [cI, l=$i_x$] ($(O)+(0.0,0.0)$)
        to [I, l=1<ampere>] ($(C)+(-0.2,0.0)$) to ($(C)+(0.0,0.0)$);
    draw[-stealth] ($(O)+(-0.06,-0.06)$) to node[left]{$i_x$} ($(O)+(-0.06,-0.3)$);
    end{tikzpicture}} %EoS
end{adjustbox}


question
begin{minipage}[t]{0.65linewidth}
    Consider the network graph shown in the figure, which of the following is NOT a textit{tree} of this graph?
    begin{choices}
        begin{multichoices}
            choice
                bla bla
            choice
                bla bla
        end{multichoices}par
        begin{multichoices}
            choice
                bla bla
            CorrectChoice
                bla bla
        end{multichoices}
    end{choices}
end{minipage}hfill%
begin{adjustbox}{minipage={0.30linewidth},valign=t}
    includegraphics[width=linewidth]{example-image}
end{adjustbox}


question
begin{minipage}[t]{0.65linewidth}
    Consider the network graph shown in the figure, which of the following is NOT a textit{tree} of this graph?
    begin{choices}
        begin{multichoices}
            choice
                bla bla
            choice
                bla bla
        end{multichoices}par
        begin{multichoices}
            CorrectChoice
                bla bla
            choice
                bla bla
        end{multichoices}
    end{choices}
end{minipage}hfill%
begin{adjustbox}{minipage={0.30linewidth},valign=t}
    includegraphics[width=linewidth]{example-image}
end{adjustbox}
end{questions}
end{document}