Which of the following parasites is most commonly associated with contaminated water?

Giardia lamblia is most commonly associated with contaminated water because it is a protozoan parasite that primarily spreads through the ingestion of cysts present in contaminated drinking water. It is often found in areas with poor sanitation practices where surface water sources are used without appropriate filtration or treatment. Giardia infections can lead to gastrointestinal symptoms such as diarrhea and abdominal pain, which arise after consuming water that contains the cysts.

Other parasites listed may have different transmission routes. Ancylostoma duodenale mainly transmits through skin contact with contaminated soil. Enterobius vermicularis, known as the pinworm, typically spreads through the fecal-oral route, usually due to hand-to-mouth transfer after contact with contaminated surfaces. Schistosoma mansoni is transmitted through contact with contaminated freshwater where the larvae can penetrate the skin. Thus, Giardia lamblia stands out as the primary parasite associated specifically with contaminated water sources.