Compound A undergoes tetramerisation in the given solvent. The van't Hoff factor is

$\underset{1-\mathrm{\alpha }}{4\mathrm{A}} \rightleftharpoons \underset{\mathrm{\alpha }/4}{{\mathrm{A}}_4}$
            $\mathrm{i}=\frac{1-\mathrm{\alpha }+{\displaystyle \frac{\mathrm{\alpha }}{4}}}{1}=1-\frac{3}{4}\mathrm{\alpha }$
$\mathrm{\alpha }=$degree of association = 1, i.e. 100% association
                     $i=1-\frac{3}{4}=0.25$