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The biceps brachii is the muscle that flexes and supinates the forearm and is prominently located on the anterior portion of the upper arm. It consists of two heads: the long head and the short head, both of which converge to form a single muscle belly that is easily identifiable and palpable. This muscle plays a crucial role in movements involving the elbow joint, specifically flexing the forearm at the elbow and assisting in the supination of the forearm when the arm is in a flexed position. The biceps brachii's anatomical positioning and its function make it a key muscle in upper limb biomechanics.

The other muscles listed do not embody the specific combination of functions that the biceps brachii does. For instance, the flexor radialis primarily functions as a wrist flexor rather than affecting the forearm significantly in terms of supination. The brachioradialis does contribute to flexing the elbow but is less involved in supination compared to the biceps brachii. The teres major, while it is a muscle of the shoulder, does not play a role in forearm movement at all. Therefore, the biceps brachii is clearly the muscle that fulfills the criteria described in the