Why does a reddish-brown coating form on an iron nail and the solution turn green when the nail is placed in copper sulphate solution?

Correct answer: Iron displaces copper and forms iron sulphate.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The displaced copper deposits as a reddish-brown coating on the nail, while the iron sulphate formed makes the solution green. Unlike option A, copper cannot displace iron because copper is less reactive.