Which of the following is not part of the axial skeleton?

The humerus is classified as part of the appendicular skeleton, which comprises the limbs and their attachments to the axial skeleton. Specifically, the appendicular skeleton includes the bones of the arms and legs, as well as the girdles that connect them to the axial skeleton. In contrast, the axial skeleton consists of the skull, vertebral column, ribs, and sternum, which serve to protect vital organs and support the structure of the body. Understanding this classification helps to identify the humerus as distinct from the components of the axial skeleton, making it the correct choice for this question.