Assertion : The mantle of pearl oyster is three - layered.
Reason : Nacre- secreting cells are present just below the nacreous layer .

Options

A. If both assertion & reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both assertion & Reason are true but the reason is not the correct explanation of the assertion, them mark (2).
C. If assertion is true statement but reason is false, them mark (3).
D. If both assertion and reason are false statements, them mark (4).

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "The mantle of pearl oyster is three-layered."
- Pearl oysters belong to the phylum Mollusca, which is characterized by a body divided into three main parts: head, muscular foot, and visceral hump.
- The mantle, which covers the visceral hump, is indeed three-layered, consisting of the marginal layer, the pallial layer, and the central layer.

2. **Evaluating the Reason**:
- The reason states that "Nacre-secreting cells are present just below the nacreous layer."
- Nacre, also known as mother-of-pearl, is a shiny substance that lines the inner side of the shell and forms the outer layer of pearls.
- It is true that nacreous cells are responsible for secreting nacre and are located beneath the nacreous layer.

3. **Determining the Truth of Each Statement**:
- Both the assertion and the reason are true statements.
- However, the reason provided does not serve as a justification for the assertion about the three-layered structure of the mantle.

4. **Conclusion**:
- Since both statements are true but the reason does not explain the assertion, we conclude that the answer is:
- Assertion and reason are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
- **Assertion and Reason are true, but Reason is not the correct explanation for Assertion.**