**Core Concept**
E. histolytica is a protozoan parasite that primarily infects humans, serving as its only reservoir. This is due to the parasite's complex life cycle, which involves both a human host and a fecal-oral transmission route.

**Why the Correct Answer is Right**
E. histolytica is transmitted through the fecal-oral route, where the cysts are ingested through contaminated food or water. Once ingested, the cysts undergo excystation in the small intestine, releasing the trophozoite form that colonizes the large intestine. The parasite then forms cysts again, which are shed in the feces and can infect other humans. The human host is the only reservoir for E. histolytica due to the parasite's inability to infect other animals, making humans the sole source of infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Salmonella can infect various animals, including birds, reptiles, and mammals, serving as a reservoir for these pathogens.
**Option B:** Campylobacter jejuni can infect a range of animals, including birds, cattle, and pigs, making them potential reservoirs for this bacterium.
**Option D:** Yersinia enterocolitica can infect a variety of animals, including pigs, cattle, and rodents, serving as a reservoir for this bacterium.

**Clinical Pearl / High-Yield Fact**
E. histolytica infection can be diagnosed through stool examination, and treatment typically involves antiprotozoal medications such as metronidazole. It's essential to note that E. histolytica is distinct from E. dispar, a non-pathogenic protozoan that can also be found in human stool.

**✓ Correct Answer: C. E. histolytica**