Final answer:
The axial skeleton includes the skull, vertebral column, sternum, and rib cage. Out of the options, only the skull is a part of it.
Step-by-step explanation:
The axial skeleton, one of the major parts of the human skeletal system, includes the skull, the vertebral column, the rib cage, and the sternum. Out of the options you've provided, the skull is a part of the axial skeleton. The arm is part of the appendicular skeleton, while the pectoral and pelvic girdle serve as connection points between the axial and appendicular skeletons and thus are not considered part of the axial skeleton.
Learn more about Axial Skeleton