Assertion: In imbricate aestivation, out of five petals, one is completely internal, one is completely external and in each of the remaining three petals, one margin is internal and the other is external
Reason:Ascending imbricate aestivation is found in Cassia and gulmohur.

Options

A. a) If both assertion and reason are true and reason is the correct explanation of 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 reason is false.
D. d) If assertion and reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that in imbricate aestivation, among five petals, one petal is completely internal, one is completely external, and the remaining three petals have one margin internal and the other external.
- This means that the arrangement of the petals allows for partial overlapping, where some petals are fully hidden while others are fully exposed.

2. **Analyzing the Aestivation Type**:
- Imbricate aestivation is characterized by the overlapping of petals. In this specific arrangement, one petal is entirely inside (internal), one is entirely outside (external), and the other three petals are positioned such that they partially overlap with one edge tucked in and the other edge exposed.

3. **Confirming the Assertion**:
- The assertion accurately describes the nature of imbricate aestivation. Therefore, the assertion is **true**.

4. **Understanding the Reason**:
- The reason provided states that ascending imbricate aestivation is found in Cassia and Gulmohur.
- While this statement is true, it does not explain why the assertion about imbricate aestivation is correct. The examples given do not clarify the characteristics of imbricate aestivation itself.

5. **Evaluating the Reason**:
- Although the reason mentions specific plants that exhibit ascending imbricate aestivation, it does not serve as a correct explanation for the assertion. Thus, the reason is **false** in the context of explaining the assertion.

6. **Conclusion**:
- The assertion is true, but the reason is false. Therefore, the correct conclusion is that both the assertion and reason are true, but the reason does not correctly explain the assertion.

### Final Answer:
- The assertion is true, and the reason is false. Thus, the correct option is **B**: Both assertion and reason are true, but the reason is not the correct explanation of the assertion.