The Vapour pressure of acetone at $20^{\circ}C$ 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

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