Which position is characterized by lying flat on the back?

The supine position is defined as lying flat on the back. This position is commonly used in medical settings for various procedures, examinations, and assessments because it allows for easier access to the front of the body. When in the supine position, the head, neck, and spine are aligned, which can be beneficial for maintaining proper blood circulation and respiratory function.

In contrast, the prone position involves lying flat on the stomach, which does not align with the description provided. Sim's position typically refers to a modified side-lying posture that allows for better access to certain body areas, primarily for examinations, but it is not lying flat on the back. Fowler's position is characterized by sitting up at an angle, often used to improve breathing and comfort. Thus, the supine position is the only choice that accurately describes lying flat on the back.