Among the following compounds, the one with the highest lactice energy is

We know,

Lattice Energy $\propto \frac{\mathrm{Charge}}{\mathrm{Size}}$

Now, for compounds containing ions of same charge, lattice energy is inversely proportional to the size, and thus it increases as the size of ions decreases.

In the given compounds, size of ${\mathrm{Mg}}^{2+}$ and ${\mathrm{O}}^{2-}$ is small and they both possess higher charge i.e. $+2$ and thus $\mathrm{MgO}$ has the highest lattice energy among all.

Hence, option $\mathrm{C}$ is correct.