The given reaction is:
2MnO4^- (aq) + 5SO2 (g) + 2H2O (l) → 2Mn^2+ (aq) + 5SO4^2- (aq) + 4H^+ (aq)
This is a redox reaction which is essentially a sum of two half reactions; A reduction reaction and an oxidation reaction.
Reduction half reaction: Mn changes from +7 to +2
MnO4^- + 5e- + 8H+ ↔ Mn2+ +4H2O
Oxidation half reaction: S changes from +4 to +6
SO2 + 2H2O ↔ SO4^2- + 2e- +4H+
Reduction Reaction proceeds with the gain of electrons by one of the reactants. The species that gains electrons and undergoes reduction is termed as the oxidising agent. In contrast, the species that loses electrons and undergoes oxidation is termed as the reducing agent.
Hence, in this reaction:
MnO4^- (Mn) is the oxidizing agent
SO2 (S) is the reducing agent