**Core Concept:** The small intestine is a crucial part of the gastrointestinal tract, playing a vital role in digestion, nutrient absorption, and immune system development. It is composed of various cell types, including goblet cells, Paneth cells, and enterocytes.

**Why the Correct Answer is Right:** Option D, "B cells," is incorrect because B cells are a type of lymphocyte involved in the adaptive immune response, specifically antibody production. They are not found in the lining of the small intestine, which is primarily composed of epithelial cells (enterocytes), goblet cells, and Paneth cells.

**Why Each Wrong Option is Incorrect:**

A. **Goblet cells:** These cells secrete mucus, which lines the small intestine's mucosal surface, protecting the underlying cells from harmful substances and aiding in nutrient absorption.

B. **B cells:** As mentioned above, B cells are involved in the immune response, not found in the small intestine's lining.

C. **Paneth cells:** These cells are found in the small intestine's crypts and produce antimicrobial peptides, helping protect the gut from pathogens and aid in nutrient absorption.

D. **Epithelial cells (enterocytes):** These cells line the small intestine and play a crucial role in nutrient absorption, specifically transporting glucose, amino acids, and fatty acids across the intestinal wall.

**Clinical Pearl:** The small intestine is a dynamic and multifaceted organ, containing various cell types performing essential functions like digestion, absorption, and immune system development. Adequate understanding of these cell types is crucial for understanding gastrointestinal physiology and pathology.

**Correct Answer:** D. Epithelial cells (enterocytes)