B

Step 1: Understand the concept of work being done on a system. When work is done on a system, energy is transferred to the system, which can lead to a change in potential energy.
Step 2: Distinguish between conservative and non-conservative forces.
- Conservative forces (like gravitational force) depend only on the initial and final positions and do not dissipate energy.
- Non-conservative forces (like friction) can do work that changes the total mechanical energy of the system.
Step 3: Analyze the options.
A non-conservative force doing work on the system introduces energy (by adding energy to the system), which can lead to a decrease in potential energy since the kinetic energy or other forms of energy might increase.
Step 4: Conclusion: The correct answer is Option B: A non-conservative force leads to a decrease in potential energy of the system when work is done upon it.