What is the ulna in relation to the arm's bones?

The ulna is indeed a bone opposite to the radius in the forearm. In human anatomy, the forearm consists of two long bones: the radius and the ulna. These bones run parallel to each other, with the ulna located on the side of the forearm that is opposite to the thumb (the medial side when in the standard anatomical position). The ulna is generally larger and longer than the radius and plays a crucial role in the articulation at the elbow, connecting with the humerus. The radius, on the other hand, is located on the lateral side of the forearm, aligning with the thumb. This arrangement allows for the rotational motion of the forearm, such as when turning the hand palm up or palm down. The other choices do not accurately describe the ulna's anatomical position or function.