Assertion : In a cavity within a conductor, the electric field is zero.
Reason : Charges in a conductor reside only at its surface.

Options

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

Explanation

To solve the question, we need to analyze the assertion and the reason provided:

**Assertion:** In a cavity within a conductor, the electric field is zero.
**Reason:** Charges in a conductor reside only at its surface.

### Step-by-Step Solution:

1. **Understanding the Assertion:**
- According to electrostatic principles, when a conductor is in electrostatic equilibrium, the electric field inside the conductor is zero. This includes any cavities within the conductor. Hence, the assertion is true.

2. **Understanding the Reason:**
- In a conductor, free charges (like electrons) can move. When an external electric field is applied, these charges redistribute themselves until they reach a state of equilibrium. In this state, the charges will move to the surface of the conductor, leaving no excess charge within the bulk of the conductor. Therefore, the reason is also true.

3. **Connecting the Assertion and Reason:**
- The reason explains why the assertion is true. The charges residing only on the surface of the conductor lead to the conclusion that there is no electric field within the cavity, as the induced electric fields from the surface charges cancel any internal fields.

4. **Conclusion:**
- Since both the assertion and the reason are true, and the reason correctly explains the assertion, the correct option is:
- **Option A:** Both assertion and reason are true, and the reason is the correct explanation of the assertion.

### Final Answer:
**Option A** is correct.

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