**Core Concept**
The question is testing the understanding of the neurohormonal regulation of gastric secretion, specifically the role of the vagus nerve and the G-cell in the stomach. The G-cell is responsible for secreting gastrin, a hormone that stimulates gastric acid secretion.

**Why the Correct Answer is Right**
The correct answer is **Gastrin-releasing peptide (GRP)**, also known as bombesin. GRP is a neurotransmitter released by the vagal nerve endings on the G-cell, which stimulates the release of gastrin. Gastrin then binds to its receptor on the parietal cells, leading to the secretion of gastric acid. This process is an example of a paracrine signaling mechanism, where the release of GRP from one cell type (vagal nerve endings) influences the behavior of another cell type (G-cell).

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because acetylcholine is a neurotransmitter released by the vagus nerve, but it is not the specific neurotransmitter that stimulates gastrin release from the G-cell.
* **Option B:** This is incorrect because histamine is a neurotransmitter released by the enteric nervous system, but it is not the primary neurotransmitter involved in gastrin release.
* **Option C:** This is incorrect because somatostatin is a hormone that inhibits gastrin release, rather than stimulating it.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the release of gastrin is also influenced by the presence of food in the stomach, particularly proteins and amino acids. This is why a meal can stimulate gastric acid secretion.

**Correct Answer:** C. Gastrin-releasing peptide (GRP)