Smooth and cardiac muscle operates under involuntary control, whereas skeletal muscle operates under voluntary control.

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. Humerus is the long bone of the upper arm, extending from the shoulder to the elbow. While it is long and strong, it is not the longest bone in the body.
B. Ulna is one of the two forearm bones, located on the medial side (little finger side). It is relatively slender and much shorter than the femur.
C. Known as the shin bone, it is the second longest bone in the body and bears most of the body’s weight in the lower leg. However, it is still shorter than the femur.
D. The femur, or thigh bone, extends from the hip to the knee and is the longest, strongest, and heaviest bone in the human body. It supports much of the body’s weight, allows locomotion, and provides attachment points for many large muscles.