Right answer is (c) Mole fraction of solute in liquid phase
To elaborate: Relative lowering of vapor pressure is the difference in vapor pressure expressed as a fraction of original vapor pressure. Using Raoult’s law:
ΔP1 = X2 x P1
Therefore, ΔP1/P1 = X2 which implies the relative lowering is equal to mole fraction of solute in liquid phase. Since the solute is non-volatile, it cannot be present in vapor phase.