The structure of an ionic compound is expected to be cubic if $r_{\mathrm{c}}/r_{\mathrm{a}}$

The structure of an ionic compound is expected to be cubic if $r_{\mathrm{c}}/r_{\mathrm{a}}$ is greater than 0.732

Each corner of the cube-shaped unit cell in the basic cubic structure has a single lattice point. These atoms either designate the location of a single atom or a repeating pattern of atoms known as a motif across the lattice.

The ratio is found  as

 $\frac{r_c}{r_a}>0.732$

Hence option A is correct