Let the volume of f.c.c unit cell = V
$\rho_A = \large\frac{4\times M_A}{N_A. a^3}$
$\rho_B = \large\frac{8\times M_B}{N_A . a^3}$
$\large\frac{\rho_A}{\rho_B} = \large\frac{M_A}{2M_B} $
$\;\;\; = \large\frac{30}{2\times 50}$
$\;\;\; = \large\frac{30}{100}$
$\;\;\; = 0.3 $
$\rho_B = \large\frac{\rho_A }{\rho_B} = 3.33 g/cc$
Hence answer is (A)