Colligative properties of a solution are properties that depend on the concentration of solute particles in the solution, rather than the identity of the solute itself. These properties include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

Proportional to the number of solute particles present:
The correct answer is option A, which states that colligative properties are proportional to the number of solute particles present in the solution. This is because colligative properties depend on the concentration of the solute particles, rather than the identity of the solute.

Explanation:
When a solute is dissolved in a solvent, it dissociates into individual particles. For example, when salt (NaCl) is dissolved in water, it dissociates into Na+ and Cl- ions. Each ion acts as a separate particle in the solution. Similarly, other solutes may also dissociate or form multiple particles in solution.

The colligative properties of a solution depend on the number of solute particles present, rather than their individual properties. For example, boiling point elevation occurs because the presence of solute particles in the solution disrupts the formation of solvent vapor molecules, requiring a higher temperature to reach the boiling point.

Similarly, freezing point depression occurs because the solute particles disrupt the formation of solvent crystal lattice, preventing the solvent from freezing at its normal freezing point. Vapor pressure lowering occurs because the solute particles occupy the surface of the solvent, reducing the number of solvent molecules that can escape into the gas phase.

Osmotic pressure occurs due to the movement of solvent molecules across a semi-permeable membrane to equalize the concentration of solute particles on both sides. The osmotic pressure is directly proportional to the concentration of solute particles.

In all these cases, the colligative properties depend on the number of solute particles present in the solution, rather than their individual properties or the volume or normality of the solution. Therefore, the correct answer is option A, which states that colligative properties are proportional to the number of solute particles present.