Option 1: H2 is a reducing agent because it can donate electrons and undergo oxidation. However, it cannot act as an oxidizing agent because it does not readily accept electrons from other substances.

Option 2: H2S is a reducing agent because the sulfur in H2S can donate electrons and undergo oxidation. Like H2, it cannot act as an oxidizing agent because it does not readily accept electrons from other substances.

Option 3: SO2 is a reducing agent because the sulfur in SO2 can donate electrons and undergo oxidation. However, it can also act as an oxidizing agent because the oxygen in SO2 can accept electrons and undergo reduction.

Option 4: HI is a reducing agent because the iodine in HI can donate electrons and undergo oxidation. Iodine itself is a good oxidizing agent due to its high electronegativity.

So, the correct answer is option 3, because SO2 is the only reducing agent among the given options that can also act as an oxidizing agent.