Assertion`:-` When the muscle fibre contracts, sacromere length is reduced.
Reason `:-` Sliding of myosin filament occurs due to rotational movement of myosin head.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion .
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are false.

Answer

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "when the muscle fiber contracts, the sarcomere length is reduced."
- A sarcomere is the basic unit of a muscle fiber, consisting of thick (myosin) and thin (actin) filaments. During muscle contraction, the thin filaments slide over the thick filaments, causing the sarcomere to shorten.
- Therefore, the assertion is **true**.

2. **Understanding the Reason**:
- The reason states that "the sliding of myosin filament occurs due to rotational movement of myosin head."
- In muscle contraction, it is actually the actin filaments that slide over the myosin filaments, not the other way around. The myosin heads do pivot or rotate to pull the actin filaments, but the statement implies that the myosin itself is sliding, which is incorrect.
- Hence, the reason is **false**.

3. **Conclusion**:
- Since the assertion is true and the reason is false, the correct answer is that the assertion is true, but the reason is false.

### Final Answer:
- The assertion is true, but the reason is false.