The most powerful supinator of the arm is a) biceps brachii.
The biceps brachii muscle is primarily responsible for the supination of the forearm, which is the action of turning the palm upwards. When the biceps contracts, it pulls on the radius, causing the forearm to rotate. This muscle also plays a significant role in elbow flexion. While other muscles, like the supinator muscle, assist with supination, the biceps brachii is the most effective due to its leverage and strength in the supination motion, particularly when the elbow is flexed. In contrast, the other options listed—coracobrachialis, brachialis, and triceps—do not have supination as their primary function.