Which bone is the medial bone of the forearm?

The ulna is the correct answer because it is indeed the medial bone of the forearm. In anatomical terminology, "medial" refers to a position closer to the midline of the body. The ulna is located on the inner side of the forearm when the palm is facing forward, which is how the terms "medial" and "lateral" are typically defined in relation to limb positioning.

In contrast, the radius is the lateral bone of the forearm, positioned on the outer side of the forearm. The humerus, while an important bone in the arm, is actually located in the upper arm, connecting to the shoulder and forming the elbow joint. The sternum is a bone located in the center of the chest and has no direct relevance to the structure of the forearm. Thus, the ulna's position and orientation in the forearm make it the correct answer to the question.