What is the basic structural unit of bone?

Rationale:
A. Osteons are structural units of compact bone, not bone-resorbing cells.
B. Osteoblasts are bone-building cells that secrete the matrix and promote bone formation, not breakdown.
C. Osteocytes are mature bone cells derived from osteoblasts. They maintain bone tissue but do not actively break it down.
D. Osteoclasts are large, multinucleated cells responsible for bone resorption. They secrete enzymes and acids that dissolve the bone matrix, playing a vital role in bone remodeling, calcium homeostasis, and repair.

Rationale:
A. Hinge joints, like the elbow and knee, allow movement in only one plane (flexion and extension) and therefore have a limited range of motion.
B. Condyloid joints, such as the wrist, permit movement in two planes (flexion/extension and abduction/adduction) but do not allow full rotation.
C. Ball-and-socket joints, including the shoulder and hip, consist of a spherical head fitting into a cup-like cavity, allowing movement in multiple planes—flexion, extension, abduction, adduction, rotation, and circumduction—providing the greatest range of motion of all joint types.
D. Saddle joints, like the thumb carpometacarpal joint, allow movement in two planes with some rotation but still offer less overall mobility than ball-and-socket joints.