**Core Concept**
The 'D' cells of the stomach are a type of enteroendocrine cell that play a crucial role in gastric acid secretion. They are a subtype of the diffuse neuroendocrine system and are involved in the paracrine regulation of gastric secretion.

**Why the Correct Answer is Right**
'D' cells secrete somatostatin, a peptide hormone that inhibits the release of various hormones and growth factors. In the context of gastric secretion, somatostatin acts on the parietal cells to decrease the secretion of hydrochloric acid and on the G cells to decrease the secretion of gastrin. This regulatory pathway helps to maintain a delicate balance in gastric acid production. The somatostatin released by 'D' cells acts through somatostatin receptors, specifically SSTR2 and SSTR5, which are G protein-coupled receptors that inhibit adenylate cyclase activity and reduce cyclic AMP levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrin - Incorrect because gastrin is secreted by G cells, not 'D' cells.
**Option B:** Histamine - Incorrect because histamine is secreted by ECL cells, not 'D' cells.
**Option C:** Secretin - Incorrect because secretin is secreted by S cells in the duodenum, not 'D' cells in the stomach.

**Clinical Pearl / High-Yield Fact**
Somatostatin has a broad range of effects on various physiological processes, including inhibition of hormone secretion, suppression of cell proliferation, and modulation of immune responses. In the clinical setting, octreotide, a synthetic analogue of somatostatin, is used to treat various conditions, including acromegaly, carcinoid syndrome, and diarrhea associated with carcinoid tumors.

**Correct Answer:** D. Somatostatin.