$AB$ is an ionic solid. The ionic radii of $A^{+}$ and $B^{-}$ are respectively $r_c$ and $r_a$. Lattice energy of AB is proportional to :

Question

$$\begin {array} {1 1} (a)\;\frac{r_c}{r_a} & \quad (b)\;(r_c+r_a) \\ (c)\;\frac{r_a}{r_c} & \quad (d)\;\frac{1}{r_c+r_a)} \end {array}$$