Which of the following terms would accurately describe a person lying flat on their back?

The term "supine" refers specifically to a person lying flat on their back, facing upwards. This position is commonly used in various medical examinations and procedures, as it provides full access to the front of the body and is often more comfortable for patients during assessments or treatments.

In contrast, "prone" describes a position where a person lies face down, which is obviously not applicable in this context. The term "recumbent" generally means lying down but can refer to any position rather than specifically being on the back, while "sitting" describes an upright position, which is also not relevant here. Therefore, "supine" is the most accurate term to describe a person lying flat on their back, making it the correct choice.