In HCP lattice, effective number of atoms present in unit cell
$=6$

Thus, the number of anions per unit cell $=6$

The number of octahedral voids is equal to the effective number of atoms in the unit cell.

According to the question, cations are occupying $75\%$ of the octahedral voids,

Thus, the number of cations per unit cell $=6\times \frac{75}{100}=\frac{9}{2}$

The formula of the ionic compound represents the simplest ratio of the ions present in it. The cation is written first and then the anion.

Thus, the formula of the given compound$={\mathrm{C}}_{\frac{9}{2}}{\mathrm{A}}_6\Rightarrow {\mathrm{C}}_9{\mathrm{A}}_{12}={\mathrm{C}}_3{\mathrm{A}}_4$

Hexagonal close packed (hcp) refers to layers of spheres packed so that spheres in alternating layers overlie one another.