JEE Main · 2021 · 31 Aug 2021 · Shift 2 · Solid State

In a cubic close-packed array the anions B give $4$ per unit cell, and the number of octahedral voids equals the number of packing atoms, so there are $4$ octahedral sites. Filling all octahedral sites with cations A places $4$ A per cell. The ratio A : B is $4:4=1:1$, so the empirical formula is $\mathrm{AB}$ and $x=1$.