Which position describes a person lying face down?

The correct choice describes a person lying face down as "prone." This term is commonly used in medical and anatomical contexts to specify the position of the body. In the prone position, the individual is flat on their stomach, which is significant in various medical assessments and procedures, such as during resuscitation or when assessing injuries to the back.

The other positions listed have distinct definitions: "supine" refers to a person lying on their back, which contrasts with the prone position. "Upright" indicates a standing or sitting position, so it does not relate to lying down at all. "Recumbent" implies lying down but does not specify whether the person is on their front or back, making it less precise in this context. Thus, "prone" is the most accurate description of a person lying face down.