**Core Concept**
Cholecystokinin (CCK) is a hormone released by the duodenal mucosa in response to fat and protein ingestion. It plays a crucial role in the digestion of fats and proteins by regulating the release of digestive enzymes and bile from the pancreas and gallbladder, respectively.

**Why the Correct Answer is Right**
CCK stimulates the contraction of the gallbladder, leading to the release of bile into the bile duct and subsequent flow into the small intestine. This facilitates the emulsification of fats, allowing lipases to act on them. Additionally, CCK promotes the secretion of pancreatic juice rich in enzymes, including amylase, lipase, and trypsinogen, which break down carbohydrates, fats, and proteins, respectively. Furthermore, CCK increases the secretion of enterokinase, an enzyme produced by the duodenal mucosa that activates trypsinogen to trypsin, a critical step in protein digestion. The synergistic action of CCK on these three processes ensures efficient digestion and absorption of nutrients.

**Why Each Wrong Option is Incorrect**
**Option A:** While CCK does stimulate the contraction of the gallbladder, this is not the only action of CCK, making it an incomplete answer.
**Option B:** Although CCK does promote the secretion of pancreatic juice rich in enzymes, this is not the sole action of CCK, and option D is more comprehensive.
**Option C:** CCK does increase the secretion of enterokinase, but this is not the only action of CCK, and option D encompasses all three actions.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that CCK levels are often elevated in conditions involving excessive fat ingestion, such as pancreatic insufficiency or gastric outlet obstruction. This can lead to symptoms like nausea, vomiting, and abdominal pain, which are often relieved by administering a CCK receptor antagonist.

**✓ Correct Answer: D. All**