Assertion (A): Sodium metal is stored under kerosene.
Reason (R ): Metallic sodium melts when exposed to air.

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

**Step-by-Step Solution:**

1. **Understanding the Assertion:**
- The assertion states that sodium metal is stored under kerosene. This is a common practice in laboratories and industries.

2. **Reason for Storage:**
- Sodium metal is highly reactive, especially with moisture and air. When sodium comes into contact with moisture (water vapor in the air), it reacts vigorously to form sodium hydroxide (NaOH) and hydrogen gas (H₂). This reaction is exothermic and can lead to fires or explosions due to the flammability of hydrogen gas.

3. **Analyzing the Reason:**
- The reason provided states that metallic sodium melts when exposed to air. This statement is misleading. While sodium does have a melting point (98°C), the primary concern when sodium is exposed to air is not melting but rather its reaction with moisture.

4. **Chemical Reaction:**
- When sodium reacts with water (even in the form of vapor), the reaction can be represented as:
\[ 2 \text{Na} + 2 \text{H}_2\text{O} \rightarrow 2 \text{NaOH} + \text{H}_2 \uparrow \]
- This shows that sodium does not simply melt; it reacts and is consumed in the process.

5. **Conclusion:**
- The assertion that sodium metal is stored under kerosene is correct because it prevents the sodium from reacting with moisture in the air. However, the reason that metallic sodium melts when exposed to air is incorrect, as it primarily reacts rather than melting.

6. **Final Evaluation:**
- Therefore, the assertion is true, and the reason is false. The correct answer is that the assertion is correct, but the reason is incorrect.