What term is best used to identify a muscle cell?

The term that best identifies a muscle cell is "muscle fiber." Muscle fibers are the basic cellular units of skeletal muscle tissue, characterized by their elongated shape and multi-nucleated structure. They contain myofibrils, which are the contractile elements within the fibers, but it is the muscle fiber itself that is the actual cell. Each muscle fiber is surrounded by a plasma membrane called the sarcolemma and contains various organelles necessary for muscle contraction.

Other options represent components or structures related to muscle tissue but do not specifically refer to the muscle cell itself. Myofibrils are bundles of actin and myosin filaments within the muscle fiber responsible for contraction. A fascicle is a grouping of muscle fibers, and a motor unit refers to a motor neuron and the muscle fibers it innervates. None of these terms correctly define the individual muscle cell as precisely as "muscle fiber" does.