Final answer:
The latissimus dorsi is the muscle that extends, adducts, and medially rotates the arm, located on the inferior part of the back.
Step-by-step explanation:
The muscle that extends, adducts, and medially rotates the arm is the latissimus dorsi. This muscle is located on the inferior part of the back and is part of the muscles that move the humerus. It inserts into the humerus through a thick connective tissue sheath known as an aponeurosis. Although the pectoralis major and teres major also contribute to these movements, the key identifying actions that are extended, adducted, and medially rotated point to the latissimus dorsi as the correct answer.