The sliding filament theory describes the mechanism that allows muscles to contract. According to this theory, myosin (a motor protein) binds to actin. The myosin then alters its configuration, resulting in a "stroke" that pulls on the actin filament and causes it to slide across the myosin filament. The overall process shortens the sarcomere structure, but does not change the actual length of either filament.

The correct answer is ATP and calcium ions. Myosin head activation to form a cross-bridge with actin requires ATP, and the cleavage of ATP to ADP + P_i contracts the myosin head and pulls the actin. Calcium is required to expose actin binding sites for myosin in conjunction with troponin. 

In the sliding filament theory, myosin heads attach to an actin filament, bend to pull the actin filaments closer together, then release, reattach, and pull again. Energy from ATP is required for the myosin head to release from the actin filament—otherwise the myosin heads would remain in the same place, and the muscle would not contract. Even though the filaments are moving, the filaments themselves never actually get shorter or longer.

When ATP stores are depleted, myosin becomes incapable of detaching from actin, and the muscle remains in a taut, flexed state. This is the cause of rigor mortis.

The myosin head will hydrolyze the . Being bound to ADP, this allows the myosin head to form crossbridges by binding to actin. As ADP detaches from the myosin head, the head will produce the power stroke motion, where the myosin heads will rotate toward the sarcomeres. The myosin head will be locked in this position, attached to the actin, until another ATP molecule comes and attaches to the myosin head. This will allow the head to detach from actin and reorient itself to complete the process again. 

Osteogenic cells are a type of stem cell that differentiate into osteoblasts, which allow bone to form. Eventually, osteoblasts will become enveloped into the bone matrix and differentiate into osteocytes. Osteoclasts have the opposite function of osteoblasts, and are responsible for the resorption of bone matrix. This releases calcium into the bloodstream by breaking down bone.

The skeletal system has a variety of functions, including protecting internal organs, storing minerals and energy molecules, and assisting in movement.

The production of heat, however, is a function of the muscular system.

Red bone marrow is found in the epiphyses, or ends of long bones. It is the site of hematopoiesis, or red blood cell development.

Yellow bone marrow is primarily composed of fat. Osteons are the functional units of bone, and house the cells that build and break down the bony matrix. Cartilage is found on the articular surfaces of bone, and helps provide support for joints.

Bone marrow is the major producer of blood cells, including most of those in the immune system.

The other answer options listed are functions of the bones in the skeletal system, but are not directly linked to the bone marrow. The skeletal system stores calcium and phosphorus, which in turn make the bones strong. Bones can complement to liver to detoxify blood by removing metals such as lead and radium. Muscles attach to bones and generate movement. The skull protects the brain. The rib cage protects the heart and lungs. 

Ligaments are made of dense connective tissue and connect bone to bone. Have you ever sprained your ankle? Chances are you partially or fully tore some ligaments in your ankle. Unfortunately, ligaments do not heal very well since there is almost no blood supply to them, and instead lay down scar tissue. Tendons connect bone to muscle. Muscles are already attached to tendons.

Osteoclasts are cells that resorb, or destroy bone, and are found in lacunae of bones. Since one of the functions of bone is storage of minerals, if the mineral content of the blood drops below the set point, osteoclasts are recruited. They break down bone, releasing the minerals into the blood. Osteoblasts do the opposite. If the levels of minerals in the blood are higher than the set point, osteoblasts will take them and convert them into bone via a process called bone formation, or ossification.