Assertion (A): Resistance is unaffected when wire is bent.
Reason (R): Resistance of wire is proportional to material resistivity

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 also false.

Explanation

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

**Step 1: Understand the Assertion (A)**
The assertion states that "Resistance is unaffected when a wire is bent."

**Step 2: Analyze the Reason (R)**
The reason states that "Resistance of wire is proportional to material resistivity." This means that the resistance \( R \) of a wire is given by the formula:
\[ R = \rho \frac{L}{A} \]
where:
- \( R \) is the resistance,
- \( \rho \) is the resistivity of the material,
- \( L \) is the length of the wire,
- \( A \) is the cross-sectional area of the wire.

**Step 3: Consider the Effect of Bending on Resistance**
When a wire is bent, the length \( L \) and the cross-sectional area \( A \) of the wire remain unchanged. Therefore, the resistance \( R \) does not change when the wire is bent.

**Step 4: Conclusion on the Assertion**
Since the bending does not affect the length or the cross-sectional area, the assertion is true.

**Step 5: Evaluate the Reason**
The reason provided states that the resistance is proportional to material resistivity. While this is true, it does not directly explain why the resistance remains unchanged when the wire is bent. The reason focuses on resistivity rather than the geometric factors (length and area) that actually determine resistance in this scenario.

**Final Conclusion:**
Both the assertion and the reason are true, but the reason is not the correct explanation for the assertion. Therefore, the correct answer is that the assertion is true, and the reason is true but does not explain the assertion.