The correct answer is sarcomere.
A sarcomere refers to the fundamental unit of striated muscle tissue. It is the reiterating unit between the two Z lines. The skeletal muscles comprise tubular muscle cells known as muscle fibers or myocytes that are produced in the procedure called myogenesis.
When the muscle fibers of a motor unit attain an impulse, it instigates a response in each sarcomere between the myosin and actin filaments. This response leads to a start of a contraction and the sliding filament theory.