The ventral branches of the spinal nerves form complex networks called plexuses. the plexus supplies the muscles and skin of the arm, forearm, and hands.

I believe the answer would be The musculocutaneous nerve innervates the flexor muscles of the arm, including the biceps brachii and brachialis muscles. ... Along the anterior of the forearm, the median and ulnar nerves supply nerve signals to the skin and to the flexor musclesof the hand and fingers.