Assertion : Angular momentum of a planet is constant about any point
Reason : For acting on the planet is a central force.

Options

A. If both Assertin and Reason are correct and Reason is the correct explanation of Assertion
B. If both Assertion and Reason are correct but Reason is not the correct explanation of Assertion
C. If Assertion is true but Reason is false
D. If Assertion is false but Reason is true

Explanation

To solve the assertion and reason question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "the angular momentum of a planet is constant about any point."
- Angular momentum (L) is defined as \( L = r \times p \), where \( r \) is the position vector from the point about which we are calculating angular momentum, and \( p \) is the linear momentum of the planet.

2. **Analyzing Angular Momentum**:
- For a planet moving in an orbit (like Earth around the Sun), the angular momentum can be affected by external forces.
- The gravitational force acting on the planet is a central force, which means it acts along the line connecting the center of mass of the planet and the Sun.

3. **Effect of Central Force**:
- A central force does not exert a torque about the center of mass of the planet. However, if we consider angular momentum about any point other than the center of mass, the gravitational force can create a torque.
- Therefore, while the angular momentum about the center of mass remains constant (due to no net external torque), the angular momentum about any arbitrary point may change due to the torque created by the gravitational force.

4. **Conclusion on Assertion**:
- Since the assertion claims that angular momentum is constant about any point, this is **incorrect**. Angular momentum is only constant about the center of mass when no external torques act on it.

5. **Understanding the Reason**:
- The reason states that "the force acting on the planet is a central force."
- This statement is **true** because the gravitational force between the planet and the Sun is indeed a central force.

6. **Final Evaluation**:
- Since the assertion is false and the reason is true, the correct conclusion is that the assertion is incorrect while the reason is correct.

### Final Answer:
- The assertion is false, and the reason is true.