Name the gas that can readily decolourise acidified ${\mathrm{KMnO}}_4$ solution

$\begin{array}{l}\mathrm{The} \mathrm{reaction} \mathrm{of} {\mathrm{KMnO}}_4 \mathrm{with} {\mathrm{SO}}_2 \mathrm{is}\\ \mathrm{K}\stackrel{+7}{\mathrm{Mn}}{\mathrm{O}}_4+\stackrel{+4}{\mathrm{S}}{\mathrm{O}}_2\to \stackrel{+2}{\mathrm{Mn}}{\mathrm{SO}}_4+\mathrm{H}\stackrel{+6}{{ }_2\mathrm{S}}{\mathrm{O}}_4+{\mathrm{K}}_2{\mathrm{SO}}_4\\ \begin{array}{ll}(\mathrm{O}.\mathrm{A}.) \mathrm{R}.\mathrm{A}& \\ \mathrm{Colourless} & \end{array}\end{array}$