Answer;
A. myofibrillar proteins in muscle.
Step-by-step explanation;
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.
Myofibrils are bundles of very fine fibers that almost fill the sarcoplasm of a muscle cell, while, Myofilaments (or filaments) are ultramicroscopic, thread-like structures found in myofibrils. In other words, the myofilaments are bundled together to make a myofibril.