An involatile solute A dimerizes in a solvent as $2A \rightleftharpoons A_2$ . If m is the molarity of solute in the solution , then the equilibrium constant of the reaction is given as

Question

$\begin{array}{1 1} \large\frac{K_b(K_bm +\Delta T_b)}{(2 \Delta T_b- K_bm)^2} \\\large\frac{K_b(K_bm -\Delta T_b)}{(2 \Delta T_b- K_bm)^2} \\ \large\frac{K_b(K_bm - \Delta T_b)}{(2 \Delta T_b- K_bm)^2} \\\large\frac{K_b(K_bm +\Delta T_b)}{(2 \Delta T_b- K_bm)^2} \end{array}$