Explanation

According to Raoult's Law, the relative lowering of vapor pressure for a solution is equal to the mole fraction of the solute. 

Raoult's Law states that the vapor pressure of a solvent in a solution is equal to the vapor pressure of the pure solvent multiplied by its mole fraction in the solution.

Therefore, the relative lowering of vapor pressure, which is the difference between the vapor pressure of the pure solvent and the vapor pressure of the solution, is directly proportional to the mole fraction of the solute.