Which of the following is a diagnostic procedure?

The correct choice is endoscopy because it is a procedure that allows direct visualization of the internal organs and cavities of the body for diagnostic purposes. By using a flexible tube with a camera (an endoscope), medical professionals can examine areas such as the gastrointestinal tract, respiratory tract, and more. This capability makes endoscopy a powerful tool for diagnosing various conditions, including ulcers, tumors, and signs of inflammation.

While ultrasound, X-ray, and CT scan are also diagnostic procedures that visualize parts of the body, they do so in different ways and without direct visual access. Ultrasound uses sound waves to create images, X-rays employ radiation to capture images of internal structures, and CT scans combine X-ray images taken from multiple angles to create cross-sectional views of the body. These methods are valuable for diagnosis but differ from the direct observation provided by endoscopy.