Final answer:
The common bile duct is directly visualized using a Choledochoscope, which is a specialized endoscope designed for examining the biliary system.
Step-by-step explanation:
Direct visualization of the common bile duct is accomplished with a C. Choledochoscope. Other listed instruments, such as the sigmoidoscope, gastroscope, and cystoscope, are used for different parts of the body, like the rectum and the urinary bladder. A choledochoscope is a specialized endoscope designed specifically for viewing the biliary system, which includes the common bile duct. The common bile duct structure is formed by the union of the common hepatic duct and the gallbladder's cystic duct. Additionally, procedures like ERCP and endoscopic ultrasound can help visualize the bile ducts non-invasively.