**Core Concept**
The human intestinal parasitic nematodes are a group of roundworms that infect the human intestine, causing a range of diseases. These parasites can infest different parts of the gastrointestinal tract, including the small bowel and large bowel. Understanding the lifecycle and habitat of each parasite is crucial for diagnosis and treatment.

**Why the Correct Answer is Right**
The correct answer is **Strongyloides stercoralis**, which primarily infests the small bowel. **Strongyloides stercoralis** is a parasitic nematode that can cause strongyloidiasis, a disease characterized by the presence of the parasite in the small intestine. The parasite's lifecycle involves the invasion of the intestinal mucosa, where it can cause damage and lead to symptoms such as diarrhea, abdominal pain, and weight loss.

**Why Each Wrong Option is Incorrect**
**Option A:** **Ascaris lumbricoides** infests the small bowel, but it can also migrate to the lungs and other parts of the body during its lifecycle.

**Option B:** **Toxocara cati** is a parasitic nematode that primarily infests the small bowel of cats, but it can also infect humans, particularly children.

**Option C:** **Trichuris trichiura** infests the large bowel, not the small bowel. It is a parasitic nematode that causes trichuriasis, a disease characterized by the presence of the parasite in the large intestine.

**Clinical Pearl / High-Yield Fact**
When diagnosing intestinal parasitic infections, it's essential to consider the patient's travel history, exposure to contaminated water or food, and the presence of symptoms such as diarrhea, abdominal pain, and weight loss.

**Correct Answer:** C. Trichuris trichiura.