Which disease is most commonly caused by Giardia lamblia in the United States?

The correct answer is C. Giardiasis.

Giardiasis is an intestinal infection caused by the Giardia lamblia parasite. It is one of the most common waterborne diseases in the United States and results in symptoms such as diarrhea, abdominal cramps, and nausea. The other options listed are caused by different pathogens and are not associated with Giardia lamblia. If you have more questions or need further explanation, feel free to ask!