Assertion :- During M phase of karyokinesis of mitosis each chromosomes is made up of two chromatids.
Reason:- Before M phase DNA content have already been doubled during S-phase.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion
C. If Assertion is True but the Reason is False
D. If both Assertion & Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that during the M phase of karyokinesis in mitosis, each chromosome is made up of two chromatids. This is true because during the M phase, chromosomes are already duplicated and consist of two identical sister chromatids joined at the centromere.

**Hint**: Recall the structure of chromosomes during mitosis and the role of sister chromatids.

2. **Understanding the Reason**: The reason provided states that before the M phase, the DNA content has already doubled during the S phase. This is also true as the S phase is the part of the cell cycle where DNA replication occurs, resulting in the duplication of chromosomes.

**Hint**: Remember the sequence of the cell cycle phases, especially the significance of the S phase.

3. **Evaluating the Relationship**: The reason explains why the assertion is true. The doubling of DNA content during the S phase is what allows each chromosome to consist of two chromatids during the M phase.

**Hint**: Think about how each phase of the cell cycle prepares the cell for the next phase.

4. **Conclusion**: Both the assertion and the 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.

**Hint**: Look for options that state both parts are true and that one explains the other.

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