The van't Hoff factor of the benzoic acid solution in benzene is 0.5. In this solution, benzoic acid:

The correct option is B forms dimer
As colligative property $\propto$ $n\left(\text{no. of moles}\right)$

Therefore, $i\left(\text{van't Hoff factor}\right)=\frac{\text{Observed number of particles}}{\text{Theoretical number of particles}}$

But $i=\frac{1}{2}$, therefore, Observed number of particles < Theoretical number of particles

Thus, the benzoic acid forms a dimer.