Which ionic solid has the greatest lattice energy?

The smallest ions will be able to approach one another most closely. They will have the closest center distances and the highest lattice energies. The top of the Periodic Table contains the tiniest ions. $\mathrm{MgO}$ has the highest lattice energy because ${\mathrm{Mg}}^{2+}$ is less massive than ${\mathrm{Ca}}^{2+}$.

Hence the option B is correct.