Corrosion of iron is essentially an electrochemical phenomenon where the cell reactions are

• A
  $Fe$ is oxidised to $Fe^{2+}$ and dissolved oxygen in water is reduced to $OH^{-}$
• B
  $Fe$ is oxidised to $Fe^{3+}$ and $H_2O$ is reduced to $O_2^{2-}$
• C
  $Fe$ is oxidised to $Fe^{2+}$ and $H_2O$ is reduced to $O_2^-$
• D
  $Fe$ is oxidised to $Fe^{2+}$ and $H_2O$ is reduced to $O_2$