Assertion : A central force is such that the force on the planet is along the vector joining the sun and the planet.
Reason : Conservation of angular momentum is valid for any central force.

Options

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

Explanation

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

### Step 1: Understanding the Assertion
The assertion states that "A central force is such that the force on the planet is along the vector joining the sun and the planet."

- A central force is defined as a force that acts along the line joining the center of two bodies. In this case, the force acting on the planet (like Earth) due to the Sun is indeed directed along the line connecting the Sun and the planet. This is true for gravitational forces, which are central forces.

### Step 2: Understanding the Reason
The reason states that "Conservation of angular momentum is valid for any central force."

- The conservation of angular momentum applies to systems where there is no net external torque acting on them. For a central force, the force acts along the line joining the two bodies, meaning the angle between the force vector and the position vector is zero. Thus, the torque (τ = r × F) is zero, leading to the conclusion that angular momentum is conserved.

### Step 3: Evaluating the Relationship Between Assertion and Reason
Now we need to evaluate whether the reason explains the assertion.

- While both the assertion and reason are true, the reason does not explain why the central force acts along the line joining the two bodies. It simply states a consequence of the nature of central forces.

### Conclusion
Both the assertion and reason are true, but the reason does not provide an explanation for the assertion. Therefore, the correct answer is option B: Both assertion and reason are true, but the reason does not explain the assertion.