The axial skeleton includes:

Choice A reason: The scapula is part of the appendicular skeleton, not the axial skeleton. Therefore, this combination is incorrect.

Choice B reason: The pelvis includes both axial and appendicular components, but the pelvic girdle is considered part of the appendicular skeleton. This makes the answer inaccurate.

Choice C reason: Both the pectoral and pelvic girdles are part of the appendicular skeleton. Including them in the axial skeleton is incorrect.

Choice D reason: Arms, legs, hands, and feet are all part of the appendicular skeleton. They are not included in the axial skeleton.

Choice E reason: This is the correct answer. The axial skeleton consists of the skull, vertebral column, and rib cage. These structures form the central axis of the body and protect vital organs such as the brain, spinal cord, and thoracic organs.

Choice A reason: The sarcomere is the smallest contractile unit of a skeletal muscle. It is composed of actin and myosin filaments arranged in a precise pattern that allows for muscle contraction. Sarcomeres are repeated along the length of a myofibril and are responsible for the striated appearance of skeletal muscle.

Choice B reason: The sarcoplasmic reticulum is a specialized form of endoplasmic reticulum in muscle cells that stores and releases calcium ions, which are essential for muscle contraction. However, it is not the contractile unit itself.

Choice C reason: The sarcolemma is the plasma membrane of a muscle fiber. It plays a role in conducting electrical signals but is not the structural unit responsible for contraction.

Choice D reason: A myofibril is a long, cylindrical organelle found within muscle cells that contains many sarcomeres. While it contributes to muscle contraction, it is composed of multiple sarcomeres and is not the fundamental unit.