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 solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the angular momentum of a satellite about a planet is constant.
- Angular momentum (L) is given by the formula:
\[ L = mvr \]
where:
- \( m \) = mass of the satellite,
- \( v \) = velocity of the satellite,
- \( r \) = distance from the planet (radius).

2. **Analyzing Angular Momentum**:
- For a satellite in orbit, the mass \( m \) remains constant.
- The satellite moves in an elliptical path, meaning that as it gets closer to the planet, its speed \( v \) increases, and as it moves away, its speed decreases.
- However, the product \( mvr \) remains constant due to the conservation of angular momentum. This is because the increase in \( v \) when \( r \) decreases (and vice versa) keeps \( L \) constant.

3. **Conclusion on Assertion (A)**:
- Therefore, the assertion is true: the angular momentum of a satellite about a planet is indeed constant.

4. **Understanding the Reason (R)**:
- The reason states that gravitational force is a central force, so its torque about the sun is zero.
- A central force is one that acts along the line connecting the center of the two bodies (in this case, the planet and the sun).

5. **Analyzing Torque**:
- Torque (\( \tau \)) is defined as:
\[ \tau = r \times F \]
where \( r \) is the position vector and \( F \) is the force.
- The angle \( \theta \) between the radius vector \( r \) and the gravitational force \( F \) is zero degrees because the force acts along the radius.

6. **Calculating Torque**:
- The formula for torque can be expressed as:
\[ \tau = rF \sin(\theta) \]
- Since \( \theta = 0 \):
\[ \sin(0) = 0 \]
- Therefore, the torque becomes:
\[ \tau = rF \cdot 0 = 0 \]

7. **Conclusion on Reason (R)**:
- Thus, the reason is also true: the gravitational force being a central force results in zero torque about the sun.

8. **Final Conclusion**:
- Since both the assertion and reason are true, and the reason correctly explains the assertion, we conclude that the answer is option 1.