Final answer:
The most likely cause of the acid-fast oocysts in the fecal sample is Cryptosporidium.
Step-by-step explanation:
The most likely cause of the acid-fast oocysts in the fecal sample is Cryptosporidium. Cryptosporidium is a protozoan parasite that causes cryptosporidiosis, an intestinal illness. It is commonly found in animals and can be transmitted through contaminated water and food.