What can be dissolved in a solvent so that the lowering in vapour pressure is equal to the mole fraction of solute?

Non volatile solute

The answer is (2) non-volatile solute.

The lowering of vapor pressure is a colligative property, which means that it depends on the number of solute particles in the solution, not on the identity of the solute particles.

Volatile solutes will have a vapor pressure of their own, which will contribute to the total vapor pressure of the solution. This means that the lowering of vapor pressure will be less than the mole fraction of the solute.

Non-volatile solutes do not have a vapor pressure of their own, so the only contribution to the total vapor pressure of the solution is from the solvent. This means that the lowering of vapor pressure will be equal to the mole fraction of the solute.

Gaseous solutes are also non-volatile, so they will also have a lowering of vapor pressure that is equal to their mole fraction.

Ionic salts are not non-volatile, so they will not have a lowering of vapor pressure that is equal to their mole fraction.

Therefore, the only option that can be dissolved in a solvent so that the lowering in vapor pressure is equal to the mole fraction of solute is a non-volatile solute.