**Core Concept**
Peg cells are a type of gastric mucosal cell that plays a crucial role in the secretion of mucus and bicarbonate. They are characterized by their distinctive shape, with short, stout projections (pegs) on their surface. The primary function of peg cells is to protect the underlying gastric epithelium from acid damage by secreting a thick layer of mucus.

**Why the Correct Answer is Right**
Peg cells are located in the surface epithelium of the stomach and are responsible for producing a large amount of mucus. This mucus layer serves as a barrier against the acidic environment of the stomach, protecting the epithelial cells from acid damage. The mucus produced by peg cells also helps to facilitate the movement of food through the stomach by reducing friction between food particles and the gastric mucosa.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because goblet cells are found in the intestinal epithelium and produce mucin, a component of mucus, but are not responsible for the distinctive peg-shaped projections seen in the stomach.
**Option B:** Incorrect because chief cells are responsible for producing pepsinogen, the precursor to pepsin, which is an enzyme involved in protein digestion in the stomach.
**Option C:** Incorrect because parietal cells are responsible for producing gastric acid (HCl) and are not associated with the production of mucus or the presence of peg cells.

**Clinical Pearl / High-Yield Fact**
Peg cells are an important component of the gastric mucosa and play a crucial role in protecting the stomach from acid damage. Damage to peg cells can lead to impaired mucus production and increased risk of gastric ulcers.

**Correct Answer:** D.