Explanation

(a) Number of atoms per unit cells is given by
$N = N_{b} + \frac{N_{f}}{2} + \frac{N_{C}}{8}$
where, $N_{b}$ is the number of atoms centered in the body of the cell, $N_{f}$ is the number of atoms centered in the face of the unit cell and $N_{C}$ is the number of atoms centered at the corner.
For fcc lattice $N_{b} = 0,\ N_{f} = 6\ $and $N_{C} = 8$
$\therefore = 0 + \frac{6}{2} + \frac{8}{8} = 3 + 1 = 4$