No. of oxygen atoms per unit cell in ccp $=4\left({\mathrm{O}}^{2-}\right)$

No. of octahedral voids per unit cell $=4\left(\text{A}{\text{l}}^{3+}\right)$

No. of Tetrahedral voids per unit cell $=8\left(\text{M}{\text{g}}^{3+}\right)$

Total negative charge due to oxygen atoms $=8$

Net charge must be zero.

$\text{m4}\left(3\right)+\text{2n}\left(8\right)+4\left(-2\right)=0$

$3\mathrm{m}+4\mathrm{n}=2$

(A) $\frac{3}{2}+\frac{4}{8}=2$ is correct.

(B) $3\times 1+4\times \frac{1}{4}=4\ne 2$ is incorrect.

(C) $3\times \frac{1}{2}+4\times \frac{1}{2}=4\ne 2$ is incorrect

(D) $3\times \frac{1}{4}+4\times \frac{1}{8}=\frac{3}{4}+\frac{2}{4}=\frac{5}{4}\ne 2$ is incorrect.