**Core Concept**
Somatostatin is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G protein-coupled somatostatin receptors. It is involved in the inhibition of the secretion of various hormones.

**Why the Correct Answer is Right**
Somatostatin is primarily produced by the delta cells in the islets of Langerhans of the pancreas. Additionally, it is also produced by neuroendocrine cells in the hypothalamus, particularly in the arctuate nucleus. The release of somatostatin is regulated by a negative feedback mechanism, which helps to control the secretion of growth hormone, thyroid-stimulating hormone, and other hormones.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, although the pancreas is involved in the production of somatostatin, it is not the primary source.
**Option B:** Incorrect because, although the hypothalamus is involved in the regulation of hormone secretion, it is not the primary site of somatostatin production.
**Option C:** Incorrect because, although the stomach is a source of various hormones, it is not a primary site of somatostatin production.

**Clinical Pearl / High-Yield Fact**
Somatostatin has a wide range of clinical applications, including the treatment of acromegaly, carcinoid syndrome, and certain types of tumors. It is also used to inhibit the release of hormones in various endocrine disorders.

**Correct Answer: D. Somatostatin is primarily produced by the delta cells in the islets of Langerhans of the pancreas.**