What is the most basic unit that makes up skeletal muscle?

The most basic unit that makes up skeletal muscle is the muscle fiber. 

Muscle fibers are long, cylindrical cells made up of rod-like structures called myofibrils, which contain the contractile proteins actin and myosin. Myofibrils are arranged in a parallel manner, with the actin and myosin filaments organized in an alternating arrangement, creating the striated appearance that’s characteristic of skeletal muscle.

Several muscle fibers are bundled together to form muscle fascicles. Fascicles are surrounded by a middle layer of connective tissue known as the perimysium. Several fascicles are bundled together to form a muscle.