Which bone is directly adjacent to the ulna?

The radius is directly adjacent to the ulna as they are both long bones located in the forearm. The ulna is located on the medial side of the forearm, while the radius is positioned laterally. They run parallel to each other and are connected at both ends by ligaments, as well as by the interosseous membrane, which provides additional stability and allows for rotational movement of the forearm, such as during pronation and supination.

Other bones mentioned, such as the scapula, femur, and tibia, are not in the same anatomical region as the ulna. The scapula is a shoulder blade located in the upper back, while the femur is the thigh bone and the tibia is one of the bones in the lower leg. These bones do not have a direct adjacency to the ulna, which is why they do not fit as the correct answer to this question.