An oxidising agent is the species that gains electrons and is itself reduced, causing oxidation of another substance. When sulphur dioxide reacts with acidified potassium permanganate, manganese is reduced from +7 in MnO_4^- to +2 in Mn^2+, while sulphur is oxidised from +4 in SO_2 to +6 in SO_4^2-. Because permanganate accepts electrons and brings about the oxidation of sulphur dioxide, potassium permanganate is the oxidising agent, and its characteristic purple colour fades as colourless Mn^2+ forms. Sulphur dioxide is incorrect because it loses electrons and is the reducing agent here. Water participates as a medium and proton source but undergoes no change in oxidation state. The potassium ion is a spectator that does not change oxidation number. This decolourisation is a standard NCERT test illustrating permanganate as a powerful oxidant in acidic medium. Plausibility check: the disappearance of the purple permanganate colour signals its reduction to Mn^2+, confirming it acted as the oxidising agent.