Final answer:
The shoulder joint exhibits horizontal adduction when the arm moves across the midline of the body in a forward direction due to the medial rotation of the humerus.
Step-by-step explanation:
The shoulder joint exhibits horizontal adduction when the arm moves across the midline of the body in a forward direction. This movement is achieved by the medial rotation of the humerus and is a part of the glenohumeral joint's multiaxial ball-and-socket motion. As a result, the correct option is c) Moves medially.