One mole of sugar is dissolved in three moles of water at 298 K. The relative lowering of vapor pressure is

Relative lowering in vapour pressure = Mole fraction of solute
$\begin{array}{l}\therefore {\mathrm{x}}_{\text{solue }}=\frac{{\mathrm{n}}_{\text{solute }}}{{\mathrm{n}}_{\text{solute }}+{\mathrm{n}}_{\text{solvent }}}\\ =\frac{1}{1+3}=\frac{1}{4}=0.25\end{array}$