**Core Concept**
The common bile duct is a critical structure that plays a crucial role in the biliary system. It is formed by the junction of the right and left hepatic ducts and runs downward to join the pancreatic duct at the ampulla of Vater, emptying into the second part of the duodenum. The ampulla of Vater is a narrow, funnel-shaped area where the bile and pancreatic juices enter the duodenum.

**Why the Correct Answer is Right**
The ampulla of Vater is the most common site for gallstone impaction due to its narrow diameter and the convergence of the common bile duct and pancreatic duct. This area is particularly susceptible to obstruction because of the increased pressure and resistance to flow. The narrowing at the ampulla of Vater, combined with the presence of a gallstone, can lead to a complete blockage of the bile flow, resulting in severe abdominal pain, jaundice, and potentially life-threatening complications.

**Why Each Wrong Option is Incorrect**
**Option A:** The supra duodenal part of the common bile duct is not typically associated with gallstone impaction. This area is located proximal to the duodenum and is less likely to experience the increased pressure and resistance that contributes to stone impaction.
**Option B:** The retro duodenal part of the common bile duct is also not commonly associated with gallstone impaction. This area is located behind the duodenum and is not a typical site for obstruction.
**Option D:** The common hepatic duct is the junction of the right and left hepatic ducts, which form the common bile duct. While it is a critical area for bile flow, it is not the most common site for gallstone impaction.

**Clinical Pearl / High-Yield Fact**
The ampulla of Vater is a narrow, funnel-shaped area that is prone to gallstone impaction due to its anatomical structure and the convergence of the common bile duct and pancreatic duct. This knowledge is essential for surgeons and physicians to diagnose and manage gallstone disease effectively.

**✓ Correct Answer: C. Ampulla of vater**