When a non-zero volatile solute is dissolved in a solvent, the relative lowering of vapour pressure is equal to:

Molar fraction of solute.

The correct option is (1) Molar fraction of solute.

The relative lowering of vapor pressure is given by the equation:

\[ \frac{{P_{B0} - P_t}}{{P_{B0}}} = x_A \]

where \( P_{B0} \) is the vapor pressure of the pure solvent, \( P_t \) is the vapor pressure of the solution, and \( x_A \) is the mole fraction of the solute (component A).

Therefore, the relative lowering of vapor pressure is equal to the mole fraction of the solute (\( x_A \)), which corresponds to option (1).