Which of the following is used to visualize the interior of hollow organs?

Endoscopy is the correct choice as it specifically refers to a procedure that utilizes an endoscope to visualize the interior of hollow organs and cavities in the body. This technique allows healthcare providers to examine areas such as the gastrointestinal tract, respiratory tract, and other hollow organs for diagnostic purposes or therapeutic interventions. The endoscope is a flexible tube with a light and camera at the end, enabling real-time images and videos of the internal structures.

Ultrasound employs sound waves to create images of internal organs, but it does not provide the direct visualization necessary for detailed observation. Bronchoscopy is a specific type of endoscopy focused on examining the airways and lungs, and while it is effective for this purpose, it is not as generalized as the term 'endoscopy.' A PET scan is a type of imaging test that can detect metabolic activity and is commonly used in cancer diagnosis, but it does not visualize the interior of hollow organs directly. Therefore, endoscopy is the most encompassing and accurate choice for visualizing the interior of hollow organs.