Moles of solute, nB=0.25
Moles of solvent, nA=1
Mole fraction of solvent, XA=nA+nBnA=1+0.251=1.251=0.8
According to Raoult's law, the vapor pressure of the solution is given by Ps=P0XA
Ps=0.8P0=80% of P0
Thus, the vapor pressure of the solution is 80% of the vapor pressure of the pure solvent.
Answer: 80%