Which subunit of troponin binds the …

Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. ... Some of this calcium attaches to troponin, which causes it to change shape, exposing binding sites for myosin (active sites) on the actin filaments. 

• A skeletal muscle fiber is made up of two types of filaments. They are thick filament (myosin) and thin filament (actin). Contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
• Myosin (thick) filament is also a polymerized protein. Many monomeric proteins called meromyosins constitute one thick filament. 
• Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length
•  In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments.