In which of the following reactions the hydrogen peroxide acts as a reducing agent?
\( \mathrm{Mn^{2+} + H_2O_2 \rightarrow Mn^{4+} + 2OH^-} \)
\( \mathrm{HOCl + H_2O_2 \rightarrow H_3O^+ + Cl^- + O_2} \)
\( \mathrm{PbS + 4H_2O_2 \rightarrow PbSO_4 + 4H_2O} \)
\( \mathrm{2Fe^{2+} + H_2O_2 \rightarrow 2Fe^{3+} + 2OH^-} \)