Which of the following terms best describes a patient who is lying flat on their back?

The term that best describes a patient lying flat on their back is the supine position. In this position, the person's body is horizontal and facing upwards. It is often used in various medical examinations and procedures because it allows easy access to the front of the body and is a common position for diagnostic imaging, surgical procedures, and physical examinations.

In contrast, the prone position describes a person lying flat on their stomach, which is typically used for different diagnostic purposes or procedures. The sitting position refers to someone seated upright, which does not apply here since the patient is flat on their back. The Trendelenburg position involves tilting the body with the legs elevated higher than the head, primarily used in certain medical emergencies to increase blood flow to vital organs. Therefore, the supine position is clearly the most accurate description of a patient lying flat on their back.