**Core Concept**
The contraction of the gallbladder after a fatty meal is a crucial digestive process that involves the release of specific hormones. This process is primarily mediated by the hormone cholecystokinin (CCK), which is secreted by the duodenum in response to fat and proteins.

**Why the Correct Answer is Right**
Cholecystokinin (CCK) is a peptide hormone that plays a vital role in the digestive process by stimulating the contraction of the gallbladder. When CCK binds to its receptors on the gallbladder, it triggers the release of bile into the bile duct, which then flows into the duodenum to aid in fat digestion. This process is essential for the emulsification of fats and the absorption of fat-soluble vitamins.

**Why Each Wrong Option is Incorrect**
**Option A:** Vasoactive intestinal peptide (VIP) is a hormone that inhibits gallbladder contraction and has various other effects on the digestive system, making it an incorrect choice.

**Option B:** Secretin is a hormone released in response to acidic chyme in the duodenum, which stimulates the pancreas to release bicarbonate-rich secretions to neutralize the pH. It does not directly stimulate gallbladder contraction.

**Option C:** Gastrin is a hormone released by the stomach that stimulates gastric acid secretion and gastric motility but does not play a significant role in gallbladder contraction.

**Clinical Pearl / High-Yield Fact**
Cholecystokinin (CCK) is also known as the "hunger hormone" due to its role in regulating appetite and satiety. It is released in response to food intake and helps to reduce hunger by signaling the brain that the body has adequate energy stores.

**Correct Answer: C. Cholecystokinin (CCK)**