**Core Concept**
The hepatopancreatic duct is a vital part of the biliary system, responsible for transporting bile from the liver and pancreas into the small intestine for fat digestion and absorption. The correct location of its opening is crucial for understanding the anatomy and physiology of the gastrointestinal tract.

**Why the Correct Answer is Right**
The hepatopancreatic duct, also known as the common bile duct, opens into the **duodenum**, specifically at the ampulla of Vater, a small muscular structure that serves as a valve. This allows bile to mix with pancreatic juice and enter the duodenum, facilitating fat digestion. The bile ducts from the liver and gallbladder merge with the pancreatic duct to form the hepatopancreatic ampulla, which then opens into the duodenum.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the hepatopancreatic duct does not open into the stomach. The stomach is involved in mechanical and chemical digestion, but it does not receive bile or pancreatic juice directly.
**Option B:** This option is incorrect because the hepatopancreatic duct does not open into the jejunum, the second part of the small intestine. While the jejunum is involved in nutrient absorption, it does not receive bile or pancreatic juice directly.
**Option C:** This option is incorrect because the hepatopancreatic duct does not open into the colon, the large intestine. The colon is primarily involved in water and electrolyte absorption, and it does not receive bile or pancreatic juice.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the ampulla of Vater, where the hepatopancreatic duct opens into the duodenum, is a common site for gallstones to obstruct the bile flow. This can lead to biliary colic, jaundice, and pancreatitis.

**Correct Answer: D. The duodenum.**