The correct answer is A, Giardia intestinalis.  Ascaris and Enterobius are nematodes.  Entamoeba are protozoa, but Entamoeba infections occur less frequently than Giardia infections in the United States, occurring mainly in immigrants from countries where Entamoeba infections are endemic or in travelers that have been in these countries.  Giardia is a widely distributed parasite occurring in all climates and countries.  Giardia remains one of the main causes of diarrheal outbreaks associated with drinking water in the U.S.