Which part of a gastroscope is primarily used to visualize the esophagus?

The insertion tube of a gastroscope is primarily designed for visualizing the esophagus, along with other areas of the upper gastrointestinal tract. This part of the endoscope is the long, flexible section that can be maneuvered through the mouth and down the throat to provide clear views of the esophagus, stomach, and duodenum. It contains the necessary optics and is equipped with a camera at its distal end, allowing for detailed visualization of the internal structures.

The other components mentioned serve different functions. The biopsy channel is used for taking tissue samples but does not contribute to the visualization process itself. The control body is the section of the endoscope that the operator interacts with to maneuver the scope and control its position but does not directly contribute to the visualization. The light guide connector is essential for illuminating the area being examined but is not involved in the actual visual observation. Thus, the insertion tube is the correct choice for the primary purpose of visualizing the esophagus using a gastroscope.