**Core Concept**
The ileum is the final section of the small intestine, characterized by its unique histological features and functional roles in nutrient absorption and immune function. The ileum is the primary site for the absorption of vitamin B12 and bile salts, and it also plays a crucial role in the gut-associated lymphoid tissue (GALT) system.

**Why the Correct Answer is Right**
The correct answer is related to the histological features of the ileum, specifically the presence of Peyer's patches. Peyer's patches are lymphoid follicles located in the ileal mucosa, which are responsible for the immune surveillance and sampling of antigens from the intestinal lumen. The presence of Peyer's patches in the ileum is essential for the initiation of immune responses against pathogens and the maintenance of tolerance to harmless antigens.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the jejunum, not the ileum, is the primary site for the absorption of most nutrients, including carbohydrates, proteins, and fats.
* **Option B:** This option is incorrect because the duodenum, not the ileum, is the first section of the small intestine and is primarily responsible for the neutralization of gastric acid and the absorption of bile salts.
* **Option C:** This option is incorrect because the colon, not the ileum, is primarily responsible for water absorption and the formation of feces.

**Clinical Pearl / High-Yield Fact**
The ileum is also known as the "haustra-free" region of the small intestine, meaning that it lacks the characteristic haustrations (pouches) seen in the colon.

**Correct Answer: C. The ileum is the primary site for the immune surveillance and sampling of antigens through Peyer's patches.**