**Core Concept**
The contraction of the gallbladder is influenced by various hormones and neural reflexes. Cholecystokinin (CCK) plays a crucial role in gallbladder contraction, which is essential for the release of bile into the small intestine to aid in fat digestion.

**Why the Correct Answer is Right**
CCK is released from the duodenal I cells in response to the presence of fat, protein, and amino acids in the small intestine. CCK acts on the gallbladder to cause maximum contraction, which is mediated by the muscarinic receptors and the contraction of the gallbladder smooth muscle. The contraction of the gallbladder is essential for the release of bile into the common bile duct, which then flows into the small intestine to emulsify fats. The gallbladder contraction is also associated with the relaxation of the sphincter of Oddi, allowing the bile to flow into the intestine.

**Why Each Wrong Option is Incorrect**
**Option B:** Secretin is a hormone released from the duodenal S cells in response to acidic chyme entering the small intestine. It stimulates the pancreas to release bicarbonate-rich secretions to neutralize the acidity of the chyme. However, secretin has little effect on gallbladder contraction.
**Option C:** Gastrin is a hormone released from the antral G cells in response to acidic chyme entering the stomach. It stimulates gastric acid secretion but has no significant effect on gallbladder contraction.
**Option D:** Enterogastrone is a term used to describe the inhibitory effect of the small intestine on gastric acid secretion. It is primarily mediated by CCK and secretin, but it does not directly stimulate gallbladder contraction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that CCK is not only responsible for gallbladder contraction but also stimulates pancreatic enzyme secretion and bile salt release. This hormone plays a crucial role in the digestion and absorption of fats, proteins, and carbohydrates.

✓ Correct Answer: A. CCK