**Core Concept**
The gallbladder contraction is a complex physiological process regulated by the autonomic nervous system, hormones, and local factors. The primary stimulus for gallbladder contraction is the release of cholecystokinin (CCK) from the duodenal mucosa in response to fatty food intake.

**Why the Correct Answer is Right**
Cholecystokinin acts on the gallbladder via the muscarinic receptors, specifically M3 receptors, which are coupled to Gq protein. This leads to the activation of phospholipase C (PLC), resulting in the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to form inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 releases calcium ions from the sarcoplasmic reticulum, causing muscle contraction.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because, although gastrin is a hormone released from the stomach, it primarily stimulates gastric acid secretion and has a minimal effect on gallbladder contraction.
* **Option B:** This option is incorrect because secretin, a hormone released from the duodenum, primarily stimulates pancreatic bicarbonate secretion and has a minimal effect on gallbladder contraction.
* **Option C:** This option is incorrect because gastrin-releasing peptide (GRP) is a neuropeptide that stimulates gastric acid secretion but has a minimal effect on gallbladder contraction.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the gallbladder contraction is primarily regulated by the enteric nervous system, which responds to the release of cholecystokinin from the duodenal mucosa. This complex physiological process is essential for the proper digestion and absorption of fats.

**Correct Answer: C. Cholecystokinin**