Which muscles are involved in extending the arm at the elbow joint?

The triceps muscle is primarily responsible for extending the arm at the elbow joint. This muscle is located on the back of the upper arm, and its primary function is to extend the forearm by straightening the elbow. When the triceps contract, they pull on the ulna bone, leading to the extension of the arm.

While the biceps are involved in movements of the arm, their primary function is flexion of the elbow, which creates a bending motion rather than an extension. The forearm flexors are muscles that help in flexing the wrist and fingers, not in extending the arm. The deltoids are shoulder muscles that facilitate shoulder movement, not direct elbow extension.

Therefore, the triceps muscle is the key muscle involved in extending the arm at the elbow joint, making it the correct answer.