Relative lowering of vapour pressure is a colligative property because _____________.

Colligative properties are those that depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution. Relative lowering of vapour pressure is a colligative property as Raoult’s observations established that the lowering of vapour pressure depends only on the concentration of the solute particles and it is independent of their identity.

Relative lowering of vapour pressure is equal to the mole fraction of the solute.

The vapour pressure of a solution reduces on addition of a non-volatile solute, and does not depend on the nature of the solute molecules, which is due to the colligative property. Hence option (i) is correct.

When electrolytes are dissolved in solution they are separated into ions. The vapour pressure lowering does not depend on the electrolyte itself but on the number of ions. Hence (ii) is correct.

The options (iii) and (iv) are incorrect options as they go against colligative properties.

The correct options are (i) and (ii).