Final answer:
The sarcomere is the functional unit of muscle contraction, consisting of actin and myosin filaments within the muscle fiber that enable contraction through their sliding action.
Step-by-step explanation:
The functional unit of muscle contraction is D. the sarcomere. This is because the sarcomere, which is approximately 2 µm in length, is the region from one Z-line to the next within the skeletal muscle fiber. Sarcomeres are bordered by Z-discs to which the actin myofilaments are anchored, creating a three-dimensional structure that facilitates muscle contraction.
The sarcomere comprises thinner actin filaments and thicker myosin filaments, and muscle contraction occurs when these filaments slide past each other; a process powered by ATP. Each muscle fiber contains hundreds to thousands of these sarcomeres, and as they individually contract, the entire muscle fiber shortens, leading to muscle movement.