The correct option is A Actin and accessory proteins
Muscle cells are composed of muscle fibers or myocytes. These muscle fibers are in turn composed of many chains of myofibrils. Myofibril consists of long proteins - actin, myosin and titin and other accessory proteins. All these proteins are organised into thin and thick filaments, which repeat along the length of myofibril. This pattern gives striped appearance to the muscles. Muscle contraction occurs due to actin and myosin which is present on myofibril. Thin filaments consist primarily of the protein actin, coiled with nebulin filaments. Therefore, option A is correct.