Colligative properties:
Colligative properties are physical properties of solutions that depend on the number of solute particles present, rather than the nature of the solute particles themselves. These properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

Explanation:
Vapor pressure:
- Vapor pressure is the pressure exerted by the vapor phase of a substance in equilibrium with its liquid or solid phase at a given temperature.
- In a solution, the presence of a non-volatile solute decreases the vapor pressure of the solvent.
- This decrease in vapor pressure is a colligative property because it depends on the number of solute particles, not their identity.

Depression in freezing point:
- Freezing point depression refers to the phenomenon of lowering the freezing point of a solvent when a non-volatile solute is added.
- The presence of solute particles disrupts the formation of the crystal lattice of the solvent, making it more difficult for the solvent to freeze.
- This depression in freezing point is a colligative property.

Elevation in boiling point:
- Boiling point elevation refers to the increase in boiling point of a solvent when a non-volatile solute is added.
- The presence of solute particles increases the boiling point of the solvent because the vapor pressure of the solvent needs to overcome the additional pressure exerted by the solute particles.
- This elevation in boiling point is a colligative property.

Osmotic pressure:
- Osmotic pressure is the pressure required to prevent the flow of solvent across a semipermeable membrane, when the solvent and solute are separated by the membrane.
- Osmotic pressure is directly proportional to the concentration of solute particles in the solution.
- This osmotic pressure is a colligative property.

Conclusion:
Based on the above explanations, it can be concluded that vapor pressure is not a colligative property. While the presence of a non-volatile solute does affect the vapor pressure of the solvent, vapor pressure is not solely dependent on the number of solute particles. Other factors such as intermolecular forces and temperature also influence vapor pressure. Therefore, the correct answer is option 'A' - vapor pressure.