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

Sulfur dioxide (${\text{SO}}_2$) is a strong reducing agent. It reacts with acidified potassium permanganate (${\text{KMnO}}_4$), reducing the purple $Mn(VII)$ to the colourless $Mn(II)$ ion.Reaction:
$$2{\text{MnO}}_4^{-}+5{\text{SO}}_2+2{\text{H}}_2\text{O}\to 2{\text{Mn}}^{2+}+5{\text{SO}}_4^{2-}+4{\text{H}}^{+}$$