According to the van’t Hoff theory, isotonic solutions have the same osmotic pressure at the same temperature and the same molar concentration

Let $M_2$ be the molar mass of the unknown solute

Molar concentration of urea solution =$\large\frac{50gdm^{-3}}{M_2}$

Since both the solutions are isotonic, hence, by definition, their molar concentrations are equal.

Thus $\large\frac{50g\;dm^{-3}}{M_2}=\frac{8.6g dm^{-3}}{60g mol^{-1}}$

$M_2=348.8gmol^{-1}$

Hence (A) is the correct answer.