[A]: Chromosomes are divided into heterochromatin and euchromatin part.
[R]: Heterochromatin are those regions of chromosome that remain condensed during interphase and early prophase, and rest of the non-condensed form of chromosome is called euchromatin
a. If both A and R are true and R is the correct explanation of A
b. If both A and R are true but R is not the correct explanation of A
c. If A is true and R is false
d. If both A and R are false

Options

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

Answer

To solve the question regarding the assertion (A) and reason (R) about chromosomes, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Chromosomes are divided into heterochromatin and euchromatin parts."
- This statement is true because chromosomes indeed consist of these two distinct regions. Heterochromatin is typically more condensed and less transcriptionally active, while euchromatin is less condensed and more transcriptionally active.

2. **Understanding the Reason (R)**:
- The reason states that "Heterochromatin are those regions of chromosome that remain condensed during interphase and early prophase, and the rest of the non-condensed form of chromosome is called euchromatin."
- This statement is also true. Heterochromatin remains condensed during interphase, while euchromatin is in a more relaxed state, allowing for gene expression.

3. **Analyzing the Relationship Between A and R**:
- While both A and R are true, we need to determine if R correctly explains A.
- The assertion describes the division of chromosomes into heterochromatin and euchromatin, while the reason explains the characteristics of these regions. However, R does not directly explain why chromosomes are divided into these two parts; it merely describes their states.

4. **Conclusion**:
- Since both A and R are true, but R does not serve as the correct explanation for A, the correct answer is:
**b. If both A and R are true but R is not the correct explanation of A.**