Which of the following is a colligative property?

Osmotic pressure

The correct answer is (2) Osmotic pressure.

Colligative properties are properties of a solution that depend on the number of solute particles present, rather than their specific nature. These properties include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

Surface tension (1), viscosity (3), and refractive index (4) are not colligative properties. Surface tension is the property of the surface of a liquid, viscosity is a measure of a fluid's resistance to flow, and refractive index measures how light bends when it passes through a medium.

Osmotic pressure, on the other hand, is a colligative property. It is the pressure required to prevent the flow of solvent into a more concentrated solution through a semipermeable membrane. Osmotic pressure depends on the concentration of solute particles in the solution and is related to the osmolarity or osmolality of the solution.