Decrease in freezing point of $CS_2,$ $\Delta T_f=K_f \times m$
$\Rightarrow K_f= \large\frac{\text{weight of sulphur}}{\text{molecular weight of sulphur}\times 0.1\times n}$
Atomicity of sulphur in $CS_2$ (n) is $\large\frac{K_f\times 2.56}{32\times 0.1\times 0.010}$ $= \large\frac{0.1\times 2.56}{32\times 0.1\times 0.01}$$= \large\frac{256}{32} = 8$
$\therefore$ Atomicity of sulphur(n)=8
Hence (c) is the correct answer.