The basic functional unit of a skeletal muscle fiber is the sarcomere.

Here’s why:

- A skeletal muscle fiber is composed of many myofibrils (Option 1), which are long, cylindrical organelles within the muscle cell.
- Each myofibril is made up of repeating units called sarcomeres (Option 2), arranged end-to-end along its length.
- The sarcomere is the smallest segment capable of contraction and is responsible for the muscle's ability to contract.
- Within the sarcomere, there are myofilaments (Option 3), which include thin (actin) and thick (myosin) filaments. These filaments interact to produce contraction, but the myofilaments themselves are subunits of the sarcomere, not the functional unit.
- A fascicle (Option 4) is a bundle of muscle fibers, which is a higher-level structural organization but not the functional unit of contraction.

In summary, the sarcomere is the segment between two Z-discs and represents the fundamental contractile unit of skeletal muscle, making it essential for muscle function.

Reference:
Ganong’s Review of Medical Physiology, 26th Edition, Chapter 12, Page 144