The relative lowering of vapor pressure is equal to the

mole fraction of solute

The correct answer is Option (1) → mole fraction of solute

This is directly stated by Raoult's Law for relative lowering of vapor pressure:

$ \frac{P^0-P_{s}}{P^0}=X_{\text{solute}} $

Where:

$ P^0$ = vapor pressure of pure solvent

$ P_{s}$ = vapor pressure of solution

$ X_{\text{solute}}$ = mole fraction of solute