Assertion: Material used in the construction of a standard resistance is constantan or manganin.
Reason: Temperatures coefficient of constan is very small.

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 but R is true

Answer

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

### Step-by-Step Solution:

**Step 1: Understand the Assertion**
- The assertion states that the materials used in the construction of a standard resistance are constantan or manganin.
- **Conclusion**: This assertion is true. Constantan and manganin are indeed commonly used materials for making standard resistors due to their stable resistive properties.

**Step 2: Understand the Reason**
- The reason states that the temperature coefficient of constantan is very small.
- **Conclusion**: This statement is also true. The temperature coefficient of resistance (α) for constantan is low, which means that its resistance does not change significantly with temperature variations.

**Step 3: Connecting Assertion and Reason**
- The reason explains why constantan and manganin are used for standard resistors. A low temperature coefficient ensures that the resistance remains stable across different temperatures, which is crucial for standard resistors.
- **Conclusion**: The reason is a correct explanation of the assertion.

**Step 4: Final Evaluation**
- Since both the assertion and reason are true, and the reason correctly explains the assertion, we conclude that:
- Both A (Assertion) and R (Reason) are true, and R is the correct explanation of A.

### Final Answer:
Both the assertion and reason are true, and the reason is the correct explanation of the assertion.

---