The correct answer is option 'C' - troponin.

Muscle contraction is a complex process that involves the interaction of several proteins. One of the key proteins involved in muscle contraction is troponin. Troponin is a regulatory protein that is found in skeletal and cardiac muscle cells.

- The role of ATP in muscle contraction:
ATP (adenosine triphosphate) is the energy currency of the cell. It provides energy for various cellular processes, including muscle contraction. ATP is required for muscle contraction as it powers the movement of the myosin heads, which are responsible for the sliding of the actin and myosin filaments.

- The role of ATPase enzyme:
ATPase is an enzyme that catalyzes the hydrolysis of ATP to ADP (adenosine diphosphate) and inorganic phosphate (Pi). This hydrolysis reaction releases energy that is used by the myosin heads to change their conformation and perform mechanical work. In the context of muscle contraction, ATPase is specifically referred to as myosin ATPase, as it is associated with the myosin protein.

- The location of ATPase enzyme:
The myosin ATPase enzyme is located on the myosin heads, which are part of the thick myosin filaments in muscle cells. Myosin heads interact with actin filaments during muscle contraction. Troponin, on the other hand, is a regulatory protein that is located on the actin filaments. It plays a crucial role in controlling the interaction between actin and myosin during muscle contraction.

- The role of troponin in muscle contraction:
Troponin consists of three subunits - troponin C, troponin I, and troponin T. Troponin C binds to calcium ions, troponin I inhibits the interaction between actin and myosin, and troponin T anchors troponin to the actin filament. When calcium ions are released in response to a nerve signal, they bind to troponin C, causing a conformational change in troponin. This change in troponin exposes the binding sites on the actin filament, allowing the myosin heads to bind and initiate muscle contraction.

In conclusion, the ATPase enzyme required for muscle contraction is located on the myosin heads, while troponin is a regulatory protein located on the actin filaments. Troponin plays a crucial role in controlling the interaction between actin and myosin during muscle contraction.