Final answer:
The correct answer is c. chlorine-usage in water treatment.
Step-by-step explanation:
The correct answer is c. chlorine-usage in water treatment. While the other options listed, such as remains from military explosives, runoff from farms, and discharge from gasoline tanks, are all significant sources of water pollution, chlorine-usage in water treatment is not. In fact, chlorine is often used as a disinfectant in water treatment facilities to kill harmful bacteria and other microorganisms.