Final answer:
The muscle in question is the brachialis muscle, which solely focuses on flexing the forearm at the elbow joint and is innervated by the musculocutaneous nerve.
Step-by-step explanation:
The muscle that flexes the forearm and is served by the musculocutaneous nerve, originates on the anterior surface of the humerus, and inserts on the ulna is the brachialis muscle. This muscle is found deep to the biceps brachii and is a powerful flexor of the forearm at the elbow joint. It works in conjunction with the biceps brachii and the brachioradialis, which also play roles in flexing the forearm. The brachialis, specifically, is solely responsible for this action, focusing its force on flexing the elbow without interfering with movements of the hand and wrist.