What hard material makes up the shaft of long bones and the outside surfaces of other bones?

Compact bone is the correct answer because it forms the dense outer layer of bone structures, providing strength and support. This hard material is organized into a solid structure, which is essential for withstanding the stresses placed on bones during weight-bearing activities. In long bones, such as the femur or humerus, the compact bone forms the shaft (diaphysis), which is crucial for both structural integrity and the distribution of weight and force during movement.

In contrast, the other options represent different types of bone or bone-related structures. Cancellous bone, for example, is a spongy type of bone found primarily at the ends of long bones and in the interior of other bones, playing a role in lightening the weight of the bone while still providing strength. Bone marrow is the soft tissue found within the cavities of bones, involved in the production of blood cells. Fibrous bone refers to a type of connective tissue found in some joints but does not describe a specific structure in long bones. Thus, compact bone is uniquely suited to fulfill the role of providing hardness and durability to the shafts of long bones and the surfaces of other skeletal elements.