Final answer:
Point-source pollution is the most common source of water contamination, caused by contaminants entering a waterway or water body through a single site.
Step-by-step explanation:
The most common source of water contamination is point-source pollution, which occurs when contaminants enter a waterway or water body through a single site. Examples of point-source pollution include untreated sewage, wastewater from a sewage treatment plant, and leaking underground tanks.