Which compound has the largest lattice energy ?

Magnesium Oxide $\left(\mathrm{MgO}\right)$ compound has the largest lattice energy.

Ions that are the tiniest will be able to approach one another most closely. They will have the greatest lattice energies and the shortest distance between centres. On the Periodic Table, the smallest ions are found near the top. Since $\mathrm{MgO}$ has the highest lattice energy and  ${\mathrm{Mg}}^{2+}$ is smaller than  ${\mathrm{Ca}}^{2+}$.

Hence, option C is correct.