A : Angular momentum of a satellite about a planet is constant .
R :Gravitational force is a central force so its torque about the sun is zero .

Options

A. If both Assertion & Reason are true . And the reason is the correct explanation of the assertion , then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion , then mark (2)
C. If Assertion is true statement but Reason is false , then mark (3)
D. It will move the same speed , tangentially to the spacecraft

Answer

To analyze the given statements, we need to evaluate both the assertion (A) and the reason (R) separately.

### Step 1: Understanding the Assertion (A)
The assertion states that "the angular momentum of a satellite about a planet is constant."

- **Explanation**:
- A satellite in orbit around a planet experiences a gravitational force directed towards the center of the planet. This force acts as a centripetal force, keeping the satellite in circular motion.
- The angular momentum (L) of the satellite can be expressed as:
\[ L = mvr \]
where \( m \) is the mass of the satellite, \( v \) is its tangential velocity, and \( r \) is the radius of the orbit.
- Since the gravitational force does not change the speed or the radius of the orbit (assuming a stable orbit), the angular momentum remains constant.

### Step 2: Understanding the Reason (R)
The reason states that "gravitational force is a central force, so its torque about the sun is zero."

- **Explanation**:
- A central force is one that acts along the line connecting the center of the force to the object (in this case, the gravitational force between the planet and the satellite).
- The torque (\( \tau \)) about a point is given by:
\[ \tau = r \times F \]
where \( r \) is the position vector from the point of rotation to the point of application of the force, and \( F \) is the force vector.
- For a central force like gravity, the torque about the center of the planet is zero because the force acts along the radius vector (the angle \( \theta \) between \( r \) and \( F \) is 0 degrees, making \( \sin(0) = 0 \)).
- However, the statement incorrectly mentions the sun as the reference point for torque. The torque about the sun is not necessarily zero because the satellite's motion is influenced by the sun's gravity as well.

### Conclusion
- The assertion (A) is **true**: The angular momentum of a satellite about a planet is constant.
- The reason (R) is **false**: While gravitational force is indeed a central force, the torque about the sun is not zero.

### Final Answer
The assertion is true, but the reason is false.