A: Lowering of vapour pressure depends upon concentration of solute.
R: Relative lowering of vapour pressure is a colligative property

Answer

To solve the assertion and reason question, we need to analyze both statements carefully.

### Step 1: Understand the Assertion
The assertion states that "Lowering of vapor pressure depends upon concentration of solute."

- **Explanation**: This is true. When a non-volatile solute is added to a solvent, the vapor pressure of the solvent decreases. The extent of this lowering depends on the concentration of the solute particles in the solution. The more solute particles present, the greater the lowering of the vapor pressure.

### Step 2: Understand the Reason
The reason states that "Relative lowering of vapor pressure is a colligative property."

- **Explanation**: This is also true. Colligative properties are properties that depend on the number of solute particles in a solution, not their identity. The relative lowering of vapor pressure is defined as the difference between the vapor pressure of the pure solvent and the vapor pressure of the solution, divided by the vapor pressure of the pure solvent. This relative lowering is directly related to the mole fraction of the solute, which is a measure of concentration.

### Step 3: Conclusion
Both the assertion and the reason are correct. Furthermore, the reason provides a correct explanation for the assertion.

- **Final Answer**: Both Assertion (A) and Reason (R) are correct, and R is the correct explanation of A.