Which of the following best describes the ulna?

The ulna is best described as an arm bone, specifically one of the two long bones of the forearm. It runs parallel to the radius and is located on the side opposite the thumb. The ulna plays a crucial role in the structure and function of the elbow joint and contributes to the movement of the wrist. It is larger at the elbow end, forming the olecranon, which can be felt as the bony prominence at the back of the elbow.

In the context of the question, other options describe bones belonging to different parts of the body. The leg bones, such as the femur and tibia, serve different functions than those of the arm bones. Rib bones are part of the thoracic cage protecting vital organs like the heart and lungs, while pelvic bones form the structure of the pelvis and support the weight of the upper body. Thus, identifying the ulna as an arm bone is entirely accurate and aligns with its anatomical classification.