If net external torque, about a point, acting on the system is zero, then we can surely say:

Options

A. Kinetic energy of the system remains constant
B. Mechanical energy of the system remains constant
C. Toruqe of Internal forces is zero
D. Momentum of system will remain constant

Answer

To solve the question, we need to analyze the implications of having a net external torque of zero acting on a system.

### Step-by-Step Solution:

1. **Understanding Torque**: Torque (\( \tau \)) is defined as the product of the force (\( F \)) and the distance (\( r \)) from the pivot point to the line of action of the force, mathematically expressed as \( \tau = r \times F \). If the net external torque about a point is zero, it means that the sum of all torques acting on the system about that point is zero.

2. **Implications of Zero Net External Torque**:
- When the net external torque is zero, it indicates that the rotational motion of the system is not changing. This means that the angular momentum of the system is conserved.
- Mathematically, if \( \tau_{\text{net}} = 0 \), then \( \frac{dL}{dt} = 0 \), where \( L \) is the angular momentum. This implies \( L \) is constant.

3. **Analyzing Kinetic Energy**:
- Kinetic energy (\( KE \)) of a system can change even if the net torque is zero. For example, if external forces are applied that cause linear acceleration, the kinetic energy can change despite the absence of torque.
- Therefore, we cannot conclude that the kinetic energy is constant.

4. **Mechanical Energy**:
- Mechanical energy is the sum of kinetic and potential energy. If external forces are doing work on the system, the mechanical energy can change. Hence, we cannot conclude that mechanical energy is constant either.

5. **Torque of the External Force**:
- If the net external torque is zero, it does not necessarily mean that the torque of each external force is zero. It simply means that the sum of all external torques is zero. Thus, we cannot conclude that the torque of the external force is zero.

6. **Momentum**:
- Linear momentum (\( p \)) is defined as the product of mass and velocity (\( p = mv \)). If external forces are acting on the system, the momentum can change. Therefore, we cannot conclude that momentum remains constant.

7. **Conclusion**:
- The only conclusion we can draw from the fact that the net external torque is zero is that the angular momentum of the system is conserved. Thus, the correct answer to the question is that the angular momentum of the system will remain constant.

### Final Answer:
If the net external torque about a point acting on a system is zero, then we can surely say that the angular momentum of the system is constant.