The correct answer is D, Giardia intestinalis (lamblia).  Giardia is a common infection in children.  It can be transmitted through fecal-oral contamination, and can affect absorption of fats in the gut.  Some of the symptoms (excessive mucus and fat in the stools) described are not indicative for Cyclospora cayetanensis infection.  Chilomastix mesnili and Trichomonas hominis are nonpathogenic protozoans that are commonly seen in human stool.