Final answer:
When lying on his belly, Tom's position is referred to as prone, which is a face-down orientation.
Step-by-step explanation:
If Tom is lying on his belly, his position is (C) Prone. The term "prone" refers to the body position where an individual lies face down, with the front of the body, including the chest and abdomen, in contact with the supporting surface. This contrasts with the supine position, where an individual lies on their back.
Understanding anatomical terms like prone and supine is essential in healthcare, anatomy, and various contexts where precise descriptions of body positions are crucial, such as in medical examinations or when giving instructions for physical exercises.