Final answer:
Myosin is not part of the thin filaments; it constitutes the thick filaments. Actin is the primary structural component of the thin filaments, with troponin and tropomyosin serving as regulatory proteins.
Step-by-step explanation:
All of the following proteins are part of the thin filaments except: B) Myosin. Myosin is indeed a protein associated with muscle contraction, but it is not part of the thin filaments; instead, it is the main component of the thick filaments. The proteins that do make up the thin filaments are actin, which provides binding sites for myosin and is the primary structural component, and the regulatory proteins troponin and tropomyosin. When muscle is at rest, tropomyosin blocks the binding sites on actin, preventing the interaction with myosin, and troponin helps to maintain this blockage until calcium ions bind to it, triggering conformational changes that allow muscle contraction to proceed.