Assertion: Ratio of stress and strain is always constant for a substance.
Reason : This ratio is called modulus of elasticity .

Options

A. (a) If both Assertion and Reason are true and the Reason is correct explanation of the 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 the Reason is false.
D. (d) If Assertion is false but the Reason is true.

Explanation

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the ratio of stress to strain is always constant for a substance.
- **Analysis**: This statement is not true for all conditions. The ratio of stress to strain (stress/strain) is constant only within the elastic limit of a material. Beyond this limit, the material may undergo plastic deformation, and the ratio will no longer remain constant.

2. **Understanding the Reason**:
- The reason states that this ratio is called the modulus of elasticity.
- **Analysis**: The modulus of elasticity (also known as Young's modulus for tensile stress) is defined as the ratio of stress to strain within the elastic limit. Therefore, this statement is correct.

3. **Conclusion**:
- Since the assertion is incorrect (the ratio is not always constant), but the reason is correct (the ratio is called modulus of elasticity), we conclude that the assertion is false while the reason is true.

### Final Answer:
- The assertion is false, and the reason is true. Therefore, the correct answer is that the assertion is incorrect, and the reason is correct.