Active sites on the Actin become available for binding when a actine binds to troponin b troponin binds to tropomyosin calcium binds to troponin d calcium binds to tropomyosin?

C: Calcium binds to troponin. The troponin is a filament in the actin strand, and the active site needs to be uncovered so that the myosin head can bond and therefore pull the muscle to contract it.