(B) Pale green
Iron displaces copper from CuSO₄, forming FeSO₄ (iron sulphate), which gives the solution a pale green colour.
The reaction is: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Blue colour of CuSO₄ fades and pale green FeSO₄ forms. The iron nail also turns reddish-brown due to deposited copper. Examiners expect students to recall both the colour change of the solution (pale green) and its cause (FeSO₄ formation). Option (D) describes the nail, not the solution.