The fertilizers runoff and goes into the ground water system. Hence, it will reduce the groundwater pollution if we use limited fertilizers.
The repairing of leaking sewage systems will also reduce the groundwater pollution as the sewage goes underground and enters the ground water system.
The disposing of used motor oil correctly will reduce the ground water pollution as if not correctly disposed off, it will enter the groundwater system.
The burning of fossil fuels won't reduce the groundwater pollution as the burning of fuels produces gases which goes directly into the air and that mixes with water vapor and falls back to earth as acid rain and enters the ground water.
Hence, the answer: Burning fossil fuels