Final answer:
The word part that completes the medical term meaning pertaining to the middle and to the side, medi/o/______/al, is 'lateral' (Option 1).
Step-by-step explanation:
The word part that completes the medical term meaning pertaining to the middle and to the side, medi/o/______/al, is lateral (Option 1). Lateral describes the side or direction toward the side of the body, which matches the meaning of the term. The other options, superior, inferior, and anterior, do not accurately complete the medical term in this context.