The correct answer is C, Giardia intestinalis (syn. Giardia lamblia) trophozoites.  Diagnosis of giardiasis is routinely done through examination of a stool sample.  However, duodenal aspirates (or duodenal capsule technique-EnteroTest) may be useful for detection of trophozoites of Giardia intestinalis.  The parasite stages listed in answers A, B, and D would be found infrequently in duodenal aspirates.