Which muscle group is involved in the flexion of the forearm?

The biceps brachii is the primary muscle group involved in the flexion of the forearm. This muscle is located on the anterior aspect of the upper arm and is responsible for bending the elbow, which is the action described as flexion of the forearm. When the biceps brachii contracts, it pulls the forearm closer to the shoulder, effectively reducing the angle between the arm and forearm.

The trapezius muscle, located in the upper back, is primarily involved in movements of the scapula and does not contribute directly to forearm flexion. The deltoids are responsible for shoulder movements and do not play a significant role in flexing the elbow. The pectoralis major primarily aids in movements of the shoulder and does not contribute directly to the flexion of the forearm. Therefore, the involvement of the biceps brachii in forearm flexion makes it the correct answer.