**Core Concept**
Gastrin is a peptide hormone that plays a crucial role in the regulation of gastric acid secretion. It is produced by the G cells in the stomach lining and functions as a key stimulator of gastric acid production.

**Why the Correct Answer is Right**
Gastrin acts on the parietal cells in the stomach lining to stimulate the release of hydrogen ions, leading to the production of gastric acid. This process is mediated by the gastrin receptor, a G protein-coupled receptor that activates the adenylate cyclase pathway, resulting in an increase in intracellular cyclic AMP. The increased cyclic AMP then activates protein kinase A, which phosphorylates and activates the H+/K+ ATPase pump, leading to the secretion of gastric acid.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because gastrin is not produced by the chief cells, which are responsible for producing pepsinogen.
**Option B:** This option is incorrect because gastrin is not produced by the parietal cells, which are the primary site of gastric acid secretion.
**Option C:** This option is incorrect because gastrin is not produced by the enteric nervous system, although it does play a role in regulating gastric motility and secretion.

**Clinical Pearl / High-Yield Fact**
Gastrinomas, tumors that produce excessive amounts of gastrin, can lead to severe peptic ulcers and diarrhea due to the increased gastric acid production.

**Correct Answer:** G cells. Gastrin is produced by the G cells in the stomach lining.