Which part of the muscle fiber serves as the plasma membrane?

The sarcolemma is the correct answer because it specifically refers to the plasma membrane of a muscle fiber. It functions as a boundary that encloses the cell’s contents and plays a crucial role in the conduction of electrical impulses across the muscle fiber, which is essential for muscle contraction. The sarcolemma has unique properties, including the presence of ion channels, receptors, and other structures that facilitate communication and signaling within muscle cells.

The other options do not serve as the plasma membrane of muscle fibers. Sarcoplasm is the cytoplasm of muscle cells, containing various organelles and substances needed for muscle function. The sarcoplasmic reticulum is a specialized form of endoplasmic reticulum that stores calcium ions, which are vital for muscle contraction, but it is not the plasma membrane. Myofibrils are the contractile elements of muscle fibers, consisting primarily of actin and myosin filaments, but again, they are not the cell membrane itself. Thus, the sarcolemma stands out as the defining structure that serves as the plasma membrane for muscle fibers.