This question is based on osmosis and osmotic pressure. The excess pressure which must be applied to a solution to prevent the passage of solvent into it through a semi-permeable membrane. 

$$\begin{gathered} \mathrm{\pi }=\frac{{\mathrm{W}}_{\mathrm{B}}\mathrm{x} \mathrm{Rx} \mathrm{T}}{\mathrm{VX} {\mathrm{M}}_{\mathrm{B}}} \\ {\mathrm{M}}_{\mathrm{B}}=\frac{{\mathrm{W}}_{\mathrm{B}}\mathrm{x} \mathrm{R} \mathrm{x} \mathrm{T}}{\mathrm{VX} \mathrm{\pi }} \\ {\mathrm{M}}_{\mathrm{B}}=\frac{20 \mathrm{X}0.083 \mathrm{x} 273 }{3 \mathrm{x} 3.8} \\ =3 \mathrm{x} {10}^4\mathrm{g} {\mathrm{mol}}^{-1} \end{gathered}$$