In which of the following reactions the hydrogen peroxide acts as a reducing agent?

H₂O₂ acts as a reducing agent when it gets oxidized (oxygen goes from -1 to 0 or higher oxidation state).

In H₂O₂, the oxidation state of oxygen is -1.

Option A: PbS + 4H₂O₂ → PbSO₄ + 4H₂O. Here O goes from -1 in H₂O₂ to -2 in H₂O. H₂O₂ is reduced → acts as oxidizing agent.

Option B: 2Fe²⁺ + H₂O₂ → 2Fe³⁺ + 2OH⁻. Here O goes from -1 to -2. H₂O₂ is reduced → acts as oxidizing agent.

Option C: HOCl + H₂O₂ → H₃O⁺ + Cl⁻ + O₂. Here O in H₂O₂ goes from -1 to 0 (in O₂). H₂O₂ is oxidized → acts as reducing agent.

Option D: Mn²⁺ + H₂O₂ → Mn⁴⁺ + 2OH⁻. Here O goes from -1 to -2. H₂O₂ is reduced → acts as oxidizing agent.

The correct answer is Option 3: HOCl + H₂O₂ → H₃O⁺ + Cl⁻ + O₂.