Hi Vaishnavi,

The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of a substance as calculated from its mass.( As defined by Wikipedia) 

Before getting into the question, let me define one more thing, what is dimerisation?

A dimer is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, covalent or intermolecular. The process formation of dimer is called dimerisation.

Benzoic acid dimerises in Benzene, hence its Van't Hoff factor becomes 1/2 or 0.5.

Hopefully this helps.