The relative lowering of vapour pressure in case of dilute solution is directly proportional to:

Options

A. Molality
B. Molarity
C. ( c) Mole fraction
D. (d) All of these

Explanation

To solve the question, "The relative lowering of vapor pressure in case of dilute solution is directly proportional to:", we can break it down into steps.

### Step-by-Step Solution:

1. **Understanding Vapor Pressure**:
- The vapor pressure of a pure solvent is denoted as \( P_0 \).
- When a solute is added to the solvent, the vapor pressure of the solution becomes \( P_s \).

2. **Relative Lowering of Vapor Pressure**:
- The relative lowering of vapor pressure can be defined as:
\[ \text{Relative Lowering} = \frac{P_0 - P_s}{P_0} \]
- This expression gives us a measure of how much the vapor pressure decreases when a solute is added.

3. **Colligative Properties**:
- The relative lowering of vapor pressure is a colligative property, which means it depends on the number of solute particles in the solution rather than the identity of the solute.

4. **Mole Fraction of Solute**:
- The relative lowering of vapor pressure is directly proportional to the mole fraction of the solute in the solution. The mole fraction of the solute (\( X_b \)) can be expressed as:
\[ X_b = \frac{n_b}{n_a + n_b} \]
- Here, \( n_b \) is the number of moles of solute, and \( n_a \) is the number of moles of solvent.

5. **Conclusion**:
- Therefore, we can conclude that the relative lowering of vapor pressure is directly proportional to the mole fraction of the solute:
\[ \frac{P_0 - P_s}{P_0} \propto X_b \]

### Final Answer:
The relative lowering of vapor pressure in case of a dilute solution is directly proportional to the mole fraction of the solute.

---