The vapour pressures of acetone at $20^{\circ}$ is $185$ torr . When $1.2 g$ of a non- volatile substance was dissolved in 100 g of acetone at $20^{\circ}$,ITS Vapour pressure was $183$ torr. The molar mass $(g mol^{-1})$ of the substance is :

Question

$\begin{array}{1 1} 32 \\ 64 \\ 128 \\ 488 \end{array}$