## **Core Concept**
The question tests the knowledge of gastrointestinal physiology, specifically the hormonal regulation of gallbladder contraction. After a fatty meal, the body needs to release bile into the intestine to facilitate fat digestion and absorption. This process is primarily controlled by hormonal mechanisms.

## **Why the Correct Answer is Right**
The correct answer, **Cholecystokinin (CCK)**, is a hormone released by the duodenal mucosa in response to fatty acids and peptides entering from the stomach. CCK acts on the gallbladder to cause it to contract, releasing bile into the cystic duct and then into the common bile duct, and finally into the small intestine. This process is crucial for the emulsification and absorption of fats. CCK also promotes the release of pancreatic enzymes, which are essential for digestion.

## **Why Each Wrong Option is Incorrect**
- **Option A: Gastrin** - While gastrin does stimulate the secretion of gastric acid, its primary role is not in gallbladder contraction. It can have some effect on the gastrointestinal tract, but it's not the primary hormone for gallbladder contraction.
- **Option B: Secretin** - Secretin is another hormone released in response to acidic chyme entering the duodenum. It primarily stimulates the pancreas to secrete bicarbonate-rich fluid to neutralize the acidity but does not play a significant role in gallbladder contraction.
- **Option D: Gastric inhibitory polypeptide (GIP)** - GIP, also known as glucose-dependent insulinotropic peptide, is involved in glucose metabolism and inhibition of gastric acid secretion. It does not play a significant role in gallbladder contraction.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Cholecystokinin (CCK)** not only causes gallbladder contraction but also promotes pancreatic enzyme secretion and slows gastric emptying, allowing for more efficient digestion and absorption of nutrients. This makes CCK a critical hormone in the digestive process, especially after a fatty meal.

## **Correct Answer: C. Cholecystokinin.**