documentclass[12pt,a4paper]{article}
usepackage{graphicx}
usepackage[table,xcdraw]{xcolor}
renewcommand{arraystretch}{1.5}
begin{document}

begin{figure}
    centering
    includegraphics{example-image-a}
    caption{Caption}
    label{fig:my_label}
end{figure}

begin{table}
centering
begin{tabular}{rc|cc}
multicolumn{1}{l}{} & multicolumn{1}{l}{} & multicolumn{2}{|c}{Figure~ref{fig:my_label} sub-plot}                                    \ hline
Variable             & Unit                 & (a)                          & (b)                          \ hline
One          & m             & cellcolor[HTML]{FFCCC9}Vary & 50                           \
Two              & kg                    & 0.5                          & cellcolor[HTML]{FFCCC9}Vary \
Three           & Hz                   & 1000                        & 1000
end{tabular}
end{table}

end{document}

documentclass[12pt,a4paper]{article}
usepackage{graphicx,tabularx}
usepackage[table,xcdraw]{xcolor}
renewcommand{arraystretch}{1.5}
begin{document}

begin{figure}
    centering
    includegraphics{example-image-a}
    caption{Caption}
    label{fig:my_label}
end{figure}

begin{table}
centering
begin{tabularx}{.6linewidth}{rc|XX}
multicolumn{1}{l}{} & multicolumn{1}{l|}{} &   multicolumn{2}{c}{Figure~ref{fig:my_label} sub-plot}    \ hline
            Variable &         Unit          & (a)                          & (b)                          \ hline
                 One &           m           & cellcolor[HTML]{FFCCC9}Vary & 50                           \
                 Two &          kg           & 0.5                          & cellcolor[HTML]{FFCCC9}Vary \
               Three &          Hz           & 1000                         & 1000
end{tabularx}
end{table}

end{document}

[![enter code here][1]][1]


documentclass[12pt,a4paper]{article}
usepackage{graphicx}
usepackage[table,xcdraw]{xcolor}
renewcommand{arraystretch}{1.5}

begin{document}
    begin{figure}
centering
includegraphics{example-image-duck}
caption{Caption}
label{fig:my_label}
    end{figure}

    begin{table}[htb]
centering
begin{tabular}{>{raggedleft}p{4em}
                >{centering}p{4em} |
           *{2}{>{centeringarraybackslash}p{4em}}
                }
multicolumn{2}{l|}{}   & multicolumn{2}{c}{Figure~ref{fig:my_label} sub-plot}        \ hline
Variable    & Unit      & (a)                           & (b)                           \ hline
One         & m         & cellcolor[HTML]{FFCCC9}Vary  & 50                            \
Two         & kg        & 0.5                           & cellcolor[HTML]{FFCCC9}Vary  \
Three       & Hz        & 1000                          & 1000
end{tabular}
    end{table}
end{document}

documentclass[12pt,a4paper]{article}
usepackage[table,xcdraw]{xcolor}
usepackage{array}

renewcommand{arraystretch}{1.5}

newlength{templen}

begin{document}

begin{figure}[htp]
caption{Just for the number}label{fig:my_label}
end{figure}

begin{table}[htp]
centering

settowidth{templen}{Figure~ref{fig:my_label} sub-plot}
setlength{templen}{dimexpr(templen-2tabcolsep)/2}

begin{tabular}{ r c | *{2}{>{centeringarraybackslash}p{templen}} }
 & & multicolumn{2}{c}{Figure~ref{fig:my_label} sub-plot}                   \
hline
Variable & Unit & (a)                          & (b)                          \
hline
One      & m    & cellcolor[HTML]{FFCCC9}Vary & 50                           \
Two      & kg   & 0.5                          & cellcolor[HTML]{FFCCC9}Vary \
Three    & Hz   & 1000                         & 1000
end{tabular}
end{table}

end{document}