Select the correct alternative.

Options

A. Work done by kinetic friction on a body always results in a loss of its kinetic energy.
B. Work done on a body, in the motion of that body over a close loop is zero for every force in nature.
C. Total mechanical energy of a system is always conserved no matter what type of internal and ecternal forces on the body are present.
D. When total work done by a conservative force on the system is positive then the potential energy associated with this force decreases.

Explanation

To solve the question, we need to analyze each statement provided and determine if it is true or false.

### Step-by-Step Solution:

1. **Statement Analysis:**
- **Statement A:** "The work done by kinetic friction on a body always results in a loss of kinetic energy."
- **Explanation:** Kinetic friction does work on bodies in motion. However, this work can lead to a redistribution of energy between objects. For instance, if two blocks are in contact and one is moving faster, kinetic friction can increase the speed of the slower block while decreasing the speed of the faster block. Therefore, kinetic friction does not always result in a loss of kinetic energy for the entire system. This statement is **False**.

2. **Statement B:** "Work done on a body in the motion of that body over a closed loop is zero for every force in nature."
- **Explanation:** This statement is not true for non-conservative forces. For example, when friction acts on an object moving in a closed loop, it does negative work, resulting in energy loss. Therefore, the work done by non-conservative forces (like friction) over a closed loop is not zero. This statement is **False**.

3. **Statement C:** "Total mechanical energy of a system is always conserved, no matter what type of internal and external forces on the body are present."
- **Explanation:** Total mechanical energy is conserved only in the absence of non-conservative forces (like friction) and when no external work is done on the system. If there are non-conservative forces or external work, mechanical energy can change. Thus, this statement is **False**.

4. **Statement D:** "When the total work done by the conservative force on a system is positive, then the potential energy associated with this force decreases."
- **Explanation:** This statement is true. According to the work-energy principle, when work is done by a conservative force, it results in a decrease in potential energy. If the work done is positive, it means energy is being transferred to kinetic energy, leading to a decrease in potential energy. This statement is **True**.

### Conclusion:
The correct alternative is **D**.