Resting Muscle Fibre and Tropomyosin:
In a resting muscle fibre, the interaction between actin and myosin is inhibited by a protein called tropomyosin. Tropomyosin is a long, thin protein that is wrapped around the actin filaments in the muscle fibre.

Function of Tropomyosin:
Tropomyosin plays a crucial role in regulating muscle contraction. Its main function is to partially cover the myosin binding sites on actin, preventing the interaction between actin and myosin. This prevents the muscle from contracting when it is at rest.

Role of Actin and Myosin:
Actin and myosin are the two main proteins involved in muscle contraction. Actin is a thin filament, while myosin is a thick filament. When a muscle contracts, the myosin heads bind to the actin filaments, forming cross-bridges. This interaction leads to the sliding of actin filaments over the myosin filaments, resulting in muscle contraction.

Role of Troponin and Calcium:
Troponin is another protein found on the actin filaments. It has binding sites for both calcium ions and tropomyosin. When calcium ions are present, they bind to specific sites on the troponin molecule, causing a conformational change. This change in shape allows tropomyosin to move, uncovering the myosin binding sites on actin.

Explanation of the Correct Answer:
The correct answer is option 'C' - tropomyosin covers the myosin binding sites on actin in the resting muscle fibre. This means that the myosin heads are unable to bind to actin, and muscle contraction is inhibited.

When the muscle is at rest, there is no calcium present in the cytoplasm. The calcium ions are stored in the sarcoplasmic reticulum, a specialized organelle in the muscle fibre. In the absence of calcium, tropomyosin covers the myosin binding sites on actin, preventing the interaction between actin and myosin.

Summary:
In summary, in a resting muscle fibre, tropomyosin partially covers the myosin binding sites on actin, inhibiting muscle contraction. This inhibition is released when calcium ions bind to troponin, causing tropomyosin to move and uncover the myosin binding sites. This allows for the interaction between actin and myosin, leading to muscle contraction.