documentclass{article}
usepackage{xcolor}
usepackage{graphicx}
usepackage{pgfplots}
usepackage{amsmath}

usetikzlibrary{pgfplots.groupplots}


begin{document}
begin{tikzpicture}
begin{groupplot}[
    group style={
        group name=my plots,
        group size=4 by 1,
        ylabels at=edge left
    },
    %footnotesize,
    %width=3cm,
    %height=7cm,
        ticks = none,
    tickpos=left,
    ytick align=outside,
    xtick align=outside,
    enlarge x limits=false,
    xlabel=$t$,
    ylabel = $y$,
    y tick label style={
        /pgf/number format/.cd,
        fixed,
        fixed zerofill,
        precision=2,
        /tikz/.cd},
        max space between ticks=2500pt,
    xmin = -.5,
    xmax = 10,
    ymin = -.5,
    ymax = 5,
]
nextgroupplot[title=(a)]
  addplot[domain=0:2] {1};
        draw (axis cs:2,1) -- (axis cs:2,3.5) -- (axis cs: 5,3.5) -- (axis cs: 5,1);
        addplot[domain=5:10] {1};
nextgroupplot[title=(b)]
    addplot[domain=0:2] {1};
  addplot[domain=2:6,smooth] {-.65*(x - 2)*(x - 6) + 1};
    addplot[domain=6:10] {1};
nextgroupplot[title=(c)]
    addplot[domain=0:2] {1};
        draw (axis cs:2,1) -- (axis cs: 4.5,4) -- (axis cs:7,1);
        addplot[domain = 7:10] {1};
nextgroupplot[title=(d)]
        draw[->] (axis cs:0,1) -- (axis cs:2.9,1) -- (axis cs:3,4.5) -- (axis cs: 3.1,1) -- (axis cs:10,1);
    end{groupplot}
end{tikzpicture}
end{document}