Each muscle fiber is surrounded by a modified cell membrane called:

Explanation

The correct answer is the sarcolemma, which is the modified cell membrane surrounding each muscle fiber. It is essential for maintaining the structural integrity of the muscle cell and plays a vital role in the transmission of action potentials and muscle contractions. The other options, such as the sarcoplasmic reticulum, sarcomere, and myofilament, refer to different components within the muscle fiber that contribute to its function but do not serve as the membrane surrounding the fiber.