In reaction D, sulfur dioxide (SO₂) is acting as an oxidizing agent. Here's why:

Reaction D: SO₂ + 2H₂S → 3S + 2H₂O

In this reaction, sulfur dioxide (SO₂) is being reduced, while hydrogen sulfide (H₂S) is oxidized. Specifically, SO₂ gains electrons (reduction) and is transformed into elemental sulfur (S), while H₂S loses electrons (oxidation) and is converted into water (H₂O).

Thus, SO₂ acts as an oxidizing agent because it accepts electrons from H₂S, facilitating the oxidation of H₂S.