Assertion :- On adding NACl to water its vapour pressure increases.
Reason :- Addition of non-volatile solute increases the vapour pressure.

Options

A. If both assertion and reason are true and the reason is the correct explanation of the assertion.
B. If both assertion and reason are true but reason is not the correct explanation of the assertion.
C. If assertion is true but reason is false.
D. If the assertion and reason both are false.

Explanation

To analyze the assertion and reason provided in the question, we will break down the concepts related to vapor pressure, non-volatile solutes, and their effects on solutions.

### Step-by-step Solution:

1. **Understanding Vapor Pressure**:
- Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid at a given temperature. Pure solvents have a certain vapor pressure.

2. **Effect of Non-volatile Solutes**:
- Non-volatile solutes, such as NaCl (sodium chloride), do not evaporate and thus do not contribute to the vapor pressure of the solution. When a non-volatile solute is added to a solvent, it occupies space at the surface of the liquid, reducing the number of solvent molecules that can escape into the vapor phase.

3. **Raoult's Law**:
- According to Raoult's Law, the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent. When a non-volatile solute is added, the mole fraction of the solvent decreases, leading to a decrease in the vapor pressure of the solution compared to that of the pure solvent.

4. **Analyzing the Assertion**:
- The assertion states that "on adding NaCl to water, its vapor pressure increases." This is incorrect because adding NaCl (a non-volatile solute) to water will decrease the vapor pressure of the water.

5. **Analyzing the Reason**:
- The reason states that "addition of non-volatile solute increases the vapor pressure." This is also incorrect. The addition of a non-volatile solute decreases the vapor pressure of the solvent.

6. **Conclusion**:
- Both the assertion and the reason are false. Therefore, the correct answer is that the assertion is incorrect, and the reason is also incorrect.

### Final Answer:
- Both the assertion and the reason are incorrect.