(a) MgSO4 (aq) + Cu(s) → CuSO4 (aq) + Mg(s)
Copper is less reactive than magnesium, so copper cannot displace magnesium. Hence, this reaction will not occur.
(b) CuSO4 (aq) + Fe (s) → FeSO4 (aq) + Cu(s)
Iron is more reactive than copper, so it will displace copper from copper sulphate and form iron sulphate.
(c) MgSO4 (aq) + Fe (s) → FeSO4 (aq) + Mg(s)
Iron is less reactive than magensium, so iron can not displace magnesium. Hence, this reaction will not occur.