‘X’ decolourises purple coloured  acidified KMnO4 to colourless solution . ‘X’ is

CO2 , N2O5 …Central atom is in highest Oxidation  state, never act like reducing agents ;O3… is a stronger oxidant than pot. permanganate; It is used to prepare potassium permanganate;        SO2+MnO4− →H+ Mn+2+SO4−2