The reactions involved during rusting of iron are

• A

  $\mathrm{Fe}\left(\mathrm{s}\right)⟶{\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right)+2{\mathrm{e}}^{-}\left(\text{at anode }\right)$

• B

  ${\mathrm{O}}_2\left(\mathrm{g}\right)+4{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+4{\mathrm{e}}^{-}⟶2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) (\mathrm{at} \mathrm{cathode})$

• C

  $4{\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)⟶2{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+8{\mathrm{H}}^{+}$

• D

  ${\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{xH}}_2\mathrm{O}\left(\mathrm{l}\right)⟶{\mathrm{Fe}}_2{\mathrm{O}}_3\cdot {\mathrm{xH}}_2\mathrm{O}$