[A]: Chromosomes are divided into hetero- chromatin 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

Options

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 and reason about chromosomes, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "chromosomes are divided into heterochromatin and euchromatin parts."
- This is true because chromosomes can indeed be classified into these two categories based on their structure and function.

2. **Understanding the Reason**:
- The reason provided is 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 is characterized by its tightly packed structure, which remains condensed during interphase, while euchromatin is less condensed and is more accessible for transcription.

3. **Analyzing the Relationship**:
- The reason explains the assertion by providing details about the characteristics of heterochromatin and euchromatin.
- Since both the assertion and reason are true, and the reason correctly explains the assertion, we conclude that the relationship between them is valid.

4. **Choosing the Correct Option**:
- Based on the analysis, we can determine that both the assertion and reason are true, and the reason is the correct explanation of the assertion.
- Therefore, the correct answer is option A: "Both assertion and reason are true, and reason is the correct explanation of assertion."

### Final Answer:
The correct option is A: Both assertion and reason are true, and reason is the correct explanation of assertion.

---