In the rest state, a subunit of troponin masks :-A Active binding sites for actin on the myosin filaments B Active binding sites for myosin on the myosin filaments C Active binding sites for myosin on the actin filaments D Actine binding sites for actin on the actin filaments

The correct answer is C. In the rest state, a subunit of troponin masks the active binding sites for myosin on the actin filaments.

Here's a step-by-step explanation:

1. Muscle contraction is regulated by two proteins: tropomyosin and troponin. These proteins are located on the actin filament.

2. In a relaxed muscle, tropomyosin covers the myosin-binding sites on actin, preventing the myosin heads from attaching to actin and forming cross-bridges.

3. Troponin is a complex of three proteins (troponin C, troponin I, and troponin T) attached to tropomyosin. Troponin I is the inhibitory subunit; it binds to actin and holds the tropomyosin in place over the myosin-binding sites.

4. When a muscle is stimulated to contract, calcium ions bind to troponin C, causing a conformational change in the troponin-tropomyosin complex. This change moves tropomyosin away from the myosin-binding sites on actin, allowing the myosin heads to attach and initiate muscle contraction.

So, in the rest state, it's the troponin-tropomyosin complex that masks the active binding sites for myosin on the actin filaments.