ANS:D - both (b) and (c).

If the partial pressure of the solvent in the vapor phase is equal to the vapor pressure of the solvent at that temperature, then the system is said to be at its saturation temperature. At the saturation temperature, the system is in equilibrium, meaning the rate of evaporation of the solvent molecules from the liquid phase is equal to the rate of condensation of the solvent molecules from the vapor phase back into the liquid phase. This equilibrium condition leads to a constant vapor pressure of the solvent, which is equal to the partial pressure of the solvent in the vapor phase. So, the correct answer is: both (b) and (c).