The correct option is Viscosity of the solution.
Explanation
Colligative properties are properties of dilute solutions that depend solely on the number of solute particles (molecules or ions) present in a given amount of solvent, irrespective of the chemical nature or identity of the solute. These properties arise due to the presence of non-volatile solute particles affecting the solvent's physical behavior.
Analysis of Options
- Relative lowering of vapour pressure: This is a colligative property. The addition of a non-volatile solute reduces the surface area available for solvent molecules to escape, lowering the vapour pressure in proportion to the mole fraction of the solute.
- Elevation of boiling point: This is a colligative property. The boiling point of a solution is higher than that of the pure solvent because the vapour pressure is lowered, requiring a higher temperature to equal atmospheric pressure.
- Osmotic pressure: This is a colligative property. It depends on the molar concentration of the solute particles in the solution.
- Viscosity: This is not a colligative property. Viscosity is a measure of a fluid's resistance to flow and depends heavily on the nature of the solute and solvent, intermolecular forces (such as hydrogen bonding), and the size and shape of the molecules, rather than just the number of particles.
Key Takeaway: The four distinct colligative properties are relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure. Properties like viscosity, surface tension, and refractive index depend on the chemical nature of the substance and are not colligative.