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

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and Rare 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, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "In a cavity in a conductor, the electric field is zero."
- According to electrostatic principles, when a conductor is in electrostatic equilibrium, the electric field inside the conductor and any cavities within it is indeed zero. This is because any excess charge resides on the surface of the conductor, and the charges rearrange themselves in such a way that they cancel out any electric field within the conductor.

2. **Understanding the Reason (R)**:
- The reason states that "Charges in a conductor reside only at its surface."
- This statement is also true. In electrostatic equilibrium, the charges in a conductor will move to the surface and distribute themselves uniformly. There are no free charges within the bulk of the conductor, which supports the assertion that the electric field in a cavity (which is also part of the conductor) is zero.

3. **Evaluating the Relationship between A and R**:
- The reason (R) explains why the assertion (A) is true. Since there are no charges in the cavity of the conductor, the electric field must be zero.

4. **Conclusion**:
- Both the assertion (A) and the reason (R) are true, and the reason (R) correctly explains the assertion (A).

### Final Answer:
- Therefore, the correct option is: **Option 1: Both A and R are true, and R is the correct explanation of A.**