Muscle that extends, adducts, and rotates arm medially.

Latissimus dorsi.

Explanation:

Latissimus dorsi muscles is the largest muscle that lies behind the arm. This muscle is covered by the trapezius.

Latissimus dorsi muscle helps in the adduction, extension and rotates the arm medially. This muscle plays a synergistic role in the lateral flexion and extension of the arm.

Thus, the answer is latissimus dorsi muscle.

maybe

only if le monke lets me