Which position describes lying face up?

The term that describes lying face up is "supine." When a person is in the supine position, their back is flat against the surface beneath them, and they are facing upwards. This position is commonly used in various medical examinations and procedures, such as when a patient is examined, receives treatment, or rests.

In contrast, the prone position refers to lying face down, which is not applicable to this scenario. The terms "sagittal" and "midsagittal" refer to specific anatomical planes that divide the body into left and right sections, rather than describing a physical body position. Therefore, supine accurately describes the posture of lying on one's back, making it the correct choice.