(D) (b) and (d)
In (b) Ca is more reactive than Cu, so Ca displaces Cu from CuSO₄. In (d) Mg is more reactive than Zn, so Mg displaces Zn from ZnSO₄. In (a) Al cannot displace Ca (Ca is more reactive), and in (c) Cu cannot displace Fe (Fe is more reactive than Cu).
This question tests the reactivity series (displacement reactions). A metal can displace another metal from its salt solution only if it is more reactive than the metal in the salt. The reactivity order is: Mg > Ca > Al > Zn > Fe > Cu. Students often confuse Ca and Al positions — remember Ca is above Al in the series, so Al cannot displace Ca. Always check the reactivity series before predicting if a displacement reaction occurs.