A : If angular momentum of an object is constant about a point then net torque on it about that point is zero.
R : Torque is equal to the rate of change of angular momentum.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion,
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion,
C. If Assertion is true statement but Reason is false,
D. If both Assertion and Reason are false statements,

Answer

To solve the given problem, we need to analyze the statements A and R and determine their validity and relationship.

### Step-by-Step Solution:

1. **Understanding Statement A**:
- Statement A claims that if the angular momentum of an object is constant about a point, then the net torque on it about that point is zero.
- Angular momentum (L) is defined as \( L = I \omega \) for a rigid body, where \( I \) is the moment of inertia and \( \omega \) is the angular velocity.
- If angular momentum is constant, it means that \( \frac{dL}{dt} = 0 \).

2. **Understanding Statement R**:
- Statement R states that torque (\( \tau \)) is equal to the rate of change of angular momentum.
- Mathematically, this is expressed as \( \tau = \frac{dL}{dt} \).
- This means that if there is no change in angular momentum over time, the torque must be zero.

3. **Connecting A and R**:
- From Statement R, we know that torque is the rate of change of angular momentum.
- If angular momentum is constant (as stated in A), then \( \frac{dL}{dt} = 0 \).
- Therefore, substituting this into the equation for torque gives us \( \tau = 0 \).

4. **Conclusion**:
- Since both statements A and R are true, and R correctly explains why A is true, we conclude that the correct option is that both statements are true and R is the correct explanation for A.

### Final Answer:
Both Statement A and Statement R are true, and R is the correct explanation of A.

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