Which of the following is a common source of groundwater pollution?
a. oil dumping and oil spills
b. tossing candy bar wrappers into a stream
c. flushing household chemicals down the toilet
d. leaking storage containers outside factories

The correct answer is A.
The common source of groundwater pollution is oil dumping and oil spills.
 Groundwater pollution occurs when pollutants are released to the ground and they make way to the groundwater. Water pollution can also result due to presence of unwanted and minor constituent or impunity in the groundwater. It is mostly called contamination rather than pollution.
 Pollutants create contaminant plume which is within an aquifer. Pollution can occur from on-site landfills, sanitation systems, leaking sewers, petrol filling stations, effluent from wastewater treatment plants. Pollution can also occur in natural contaminants, for example, fluoride or arsenic.