It is a threadlike structure that extends from one end of the muscle fiber to another:

Option C: Myofibrils are composed of long proteins including actin, myosin, and titin, and other proteins that hold them together. These proteins are organized into thick and thin filaments called myofilaments, which repeat along the length of the myofibril in sections called sarcomeres. Option A: Sarcomere is the basic unit of striated muscle tissue. Option B: Sarcolemma is the fine transparent tubular sheath that envelops the fibers of skeletal muscles. Option D: Myofilaments are the filaments of myofibrils, constructed from proteins, principally myosin or actin.