Explanation

(d)
Lattice energy,$U = \frac{q_{1}q_{2}}{r^{2}}$
Since, interionic distances in $CaO$ and $NaCl$ are similar, (larger cation has smaller anion and vice versa) $r$ is almost the same. Therefore, lattice energy depends only on charge. Since, the magnitude of charge on $Na^{+}$ and $Cl^{-}$ ions is same $ie,$ unity and that on $Ca^{2 +}$ and $O^{2 -}\$ions is 2 each, therefore, the lattice energy of CaO is four times the lattice energy of$\ NaCl$,$\ ie$,4U