The number of carbon atoms per unit cell of diamond is:

The correct option is D $8$
Diamond has FCC lattice structure and it is built by carbon atoms. Here carbon atoms occupies the corners, face centers and also half of the tetrahedral voids.
Therefore number of carbon atoms per unit cell
$=\underset{\left(\text{Corners}\right)}{8\times \frac{1}{8}}+\underset{\left(\text{Face centered}\right)}{6\times \frac{1}{2}}+\underset{\left(\text{Tetrahedral void}\right)}{8\times \frac{1}{2}}=8$