Ans. is 'c' i.e., Myosin o Myosin is thick filament.Muscle contractiono The shortening of muscle fiber occurs due to sliding of actin filaments on myosin filament. However, there are four muscle proteins involved in the process : - Actin, Myosin, Tropomyosin, and troponin.MyosinMyosin is the protein that constitutes the thick filaments.Myosin of skeletal muscle is myosin-II.Myosin participates in the contractile mechanism and also acts as an ATPaseQActinActin is the major protein of thin filamentQ.It is the actin which slides over myosin during contraction.TropomyosinIt is the other protein of thin filamentQ.It covers the active sites (myosin binding sites) on actinQ.When Ca+2 concentration of cytoplasm (sarcoplasm) is raised, it uncovers the active sites of actin and allow the contraction So, the 'cross-bridge cycling' is switched off or on by the tropomyosin molecule which slides on the actin molecule to cover or uncover the active sites on it.TroponinIt is a protein also associated with thin filamentQ.The regulatory action of Ca+2 on tropomysin is mediated by troponin, a Ca+2 binding protein.When the cytoplasmic (sarcoplasmic) Ca+2 concentration rises Ca+2 ions bind to troponin-C.The troponin - Ca+2 complex induces changes in Troponin-I and Troponin-T, which in turn brings about a shift of tropomyosin away from the active sites of actin.When the sarcoplasmic Ca+2 concentration falls, Ca+2: dissociates from troponin-C and tropomyosin slides back on the actin filament to cover the active sites.Thus, the first event during muscle contraction, after cytoplasmic rise in Ca+2 concentration, is binding of troponin C to Ca+2 which trigers the further steps by sliding the tropomysin aw ay from the active sites of actinQ.