**Core Concept**
Argentaffinoma, also known as a carcinoid tumor, is a type of neuroendocrine tumor that originates from the argentaffin cells in the gastrointestinal tract. These cells are part of the enterochromaffin system and are responsible for producing various biologically active substances.

**Why the Correct Answer is Right**
Argentaffinomas of the ileum produce serotonin (5-HT), which is a key neurotransmitter involved in various physiological processes. The production of serotonin is mediated by the enzyme tryptophan hydroxylase, which converts the amino acid tryptophan into 5-hydroxytryptophan, which is then converted into serotonin. The serotonin produced by argentaffinomas can cause various symptoms, including flushing, diarrhea, and wheezing, due to its vasoactive and smooth muscle-stimulating properties.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as argentaffinomas do not typically produce gastrin, which is involved in acid secretion in the stomach.
**Option B:** This option is incorrect as argentaffinomas do not typically produce calcitonin, which is produced by the thyroid gland and involved in calcium homeostasis.
**Option C:** This option is incorrect as argentaffinomas do not typically produce gastrin-releasing peptide (GRP), which is a neuropeptide involved in the regulation of gastrin release.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that argentaffinomas can be slow-growing and may not produce symptoms for many years. They can be detected incidentally on imaging studies or during investigations for gastrointestinal symptoms.

**Correct Answer: C. Serotonin (5-HT)**