Assertion: Some cells enter `G_(0)` phase leasding to inactivation of cell cycle.
Reason: `G_(0)` phase occurs due to non-availability of mitogen and enrgy rich compounds.

Options

A. If both assertion and reason are true and reason is the correct explantion of assertion
B. If both assertion and reason are true but reason is not the correct explnantion of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that some cells enter the G0 phase, leading to the inactivation of the cell cycle.
- **Explanation**: The G0 phase, also known as the quiescent phase, is a state where cells exit the active cell cycle and do not proceed to divide. This can happen for various reasons, including differentiation or lack of necessary signals for division.

2. **Understanding the Reason**: The reason provided is that the G0 phase occurs due to the non-availability of mitogens and energy-rich compounds.
- **Explanation**: Mitogens are substances that stimulate cell division. When these are not available, along with a lack of energy-rich compounds (like ATP), cells may enter the G0 phase as they cannot proceed with the cell cycle.

3. **Evaluating the Truth of the Assertion and Reason**:
- The assertion is true because some cells do enter the G0 phase, leading to a halt in the cell cycle.
- The reason is also true because the lack of mitogens and energy-rich compounds can indeed lead to cells entering the G0 phase.

4. **Determining the Relationship**: We need to assess whether the reason correctly explains the assertion.
- Since the reason explains why cells enter the G0 phase and thus become inactive in the cell cycle, it serves as a correct explanation for the assertion.

5. **Conclusion**: Both the assertion and the reason are true, and the reason is a correct explanation of the assertion. Therefore, the correct option is:
- **Option A**: If both assertion and reason are true, and the reason is a correct explanation of the assertion.