Choose the correct statement

Options

A. Law of conservation of linear-momentum is valid in the presence of an external force also
B. For angular momentum of a system to remain constant, it is not necessary that external torque acting on it be zero.
C. Charge can be created and destroyed
D. A conservation law cannot be proved

Answer

To determine which statement is correct, let's analyze each statement one by one:

1. **Statement 1**: "The law of conservation of linear momentum is valid in the presence of external force also is equal to zero."
- The law of conservation of linear momentum states that if no external forces act on a system, the total linear momentum of the system remains constant. If there is an external force, the momentum can change. Therefore, this statement is **incorrect**.

2. **Statement 2**: "Angular momentum is conserved if torque is zero."
- Angular momentum is conserved when the net external torque acting on a system is zero. If there is no torque, the angular momentum remains constant. However, the statement implies that conservation of angular momentum is not necessary, which is misleading. Thus, this statement is also **incorrect**.

3. **Statement 3**: "Charge can be created and destroyed."
- According to the law of conservation of charge, charge cannot be created or destroyed; it can only be transferred from one body to another. Therefore, this statement is **incorrect**.

4. **Statement 4**: "A conservation law cannot be proved."
- Conservation laws are fundamental principles in physics that are based on empirical observations. While they cannot be "proved" in the same way as mathematical theorems, they are accepted as valid based on experimental evidence. Thus, this statement is **correct**.

### Conclusion:
The correct statement is **Statement 4**.