[A]: Sieve tube members have abundant cytoplasm but there is no nucleus.
[R]: The nucleus disintegrates during their development

Options

A. (a) If both A and R are true and R is the correct explanation of A
B. (b) If both A and R are true but R is not the correct explanation of A
C. (c) If A is true and R is false
D. (d) If both A and R are false

Answer

**Step-by-Step Solution:**

1. **Understanding the Assertion**: The assertion states that "Sieve tube members have abundant cytoplasm but there is no nucleus." This is a true statement. Sieve tube members are specialized cells found in the phloem of plants, which is responsible for the transport of sugars and other metabolic products.

2. **Identifying the Function of Sieve Tube Members**: Sieve tube members are crucial for the transport of nutrients, particularly sugars, throughout the plant. They are living cells that rely on companion cells for their metabolic functions since they lack a nucleus.

3. **Understanding the Role of Companion Cells**: Companion cells are closely associated with sieve tube members and contain a nucleus. They play a vital role in regulating the activities of sieve tube members, providing them with the necessary metabolic support.

4. **Analyzing the Reason**: The reason states that "the nucleus disintegrates during their development." This is also a true statement. During the development of sieve tube members, the nucleus breaks down, which is why these cells do not have a nucleus in their mature form.

5. **Connecting Assertion and Reason**: The reason provided explains why the assertion is true. Since the sieve tube members lose their nuclei during development, they can have abundant cytoplasm but no nucleus.

6. **Conclusion**: Both the assertion and reason are true, and the reason correctly explains the assertion. Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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

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