Final answer:
The radial tuberosity on the radius bone serves as the distal attachment site for the biceps brachii muscle, facilitating elbow flexion and forearm supination.
Step-by-step explanation:
The distal attachment site for the biceps brachii muscle is the radial tuberosity on the radius bone. The biceps brachii is a powerful muscle responsible for flexion of the elbow and supination of the forearm. The radial tuberosity is an oval-shaped, bony protuberance located on the medial side slightly below the neck of the radius. This site is critical for the muscle's function in grasping and other movements involving the forearm and hand.