Anions, ${\mathrm{A}}^{-}$ crystallize into ccp arrangement. Cations, ${\mathrm{B}}^{+}$ occupy all octahedral voids. Number of formula units per unit cell of BA is/are

${\mathrm{A}}^{-}$ form ccp arrangement.

Number of anions (${\mathrm{A}}^{-}$) per unit cell =$\left(8\times \frac{1}{8}\right)+\left(6\times \frac{1}{2}\right)=4$;

In ccp arrangement, body centre and edge centres are octahedral voids;

Number of anions (${\mathrm{B}}^{+}$) per unit cell =$\left(1\times 1\right)+\left(12\times \frac{1}{4}\right)=4$

One formula unit contains a pair of cation and anion 

$\therefore$effective number of formula units of ‘BA’ per unit cell = 4