Assertion (A): Elevation in boiling point is a colligative property.
Reason (R): Elevation in boiling point is directly proportional to molarity

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is NOT the correct explanation of A
C. A is true but R is false
D. A is false and R is True

Explanation

To solve the question regarding the assertion and reason provided, we will analyze each statement step by step.

### Step 1: Understand the Assertion
**Assertion (A):** Elevation in boiling point is a colligative property.

**Explanation:** Colligative properties are properties that depend on the number of solute particles in a solution, not on the nature of the solute. Elevation in boiling point occurs when a non-volatile solute is added to a solvent, causing the boiling point of the solution to be higher than that of the pure solvent. Therefore, the assertion is true.

### Step 2: Understand the Reason
**Reason (R):** Elevation in boiling point is directly proportional to molarity.

**Explanation:** The elevation in boiling point (ΔT_b) can be expressed by the formula:
\[ \Delta T_b = K_b \cdot m \]
where \( K_b \) is the ebullioscopic constant of the solvent and \( m \) is the molality of the solution. For very dilute solutions, molality (m) can be approximated to molarity (M), but this is not universally true for all concentrations. Thus, while the reason has some validity in specific contexts (very dilute solutions), it is not a general statement applicable to all solutions. Therefore, the reason is not entirely accurate.

### Step 3: Conclusion
- **Assertion (A)** is true.
- **Reason (R)** is not universally true; it is only true under certain conditions (very dilute solutions).

### Final Answer
Both A and R are true, but R is not the correct explanation of A.