On the basis of the electrochemical theory of aqueous corrosion,the reaction occurring at the cathode is

• A
  $O_{2(g)} + 4H^+_{(aq)} + 4e^- \to 2H_2O_{(l)}$
• B
  $Fe_{(s)} \to Fe^{2+}_{(aq)} + 2e^-$
• C
  $Fe^{2+}_{(aq)} \to Fe^{3+}_{(aq)} + e^-$
• D
  $H_{2(g)} + 2OH^-_{(aq)} \to 2H_2O_{(l)} + 2e^-$