a). Calcium ion and ATP
Explanation:
The excitation and contraction of the muscles takes place by Action potential creation.
The muscle contraction is initiated when calcium ions binds to troponin. The binding of calcium with protein complex troponin changes the configuraton to make actin sites active for ATP binding. This binding causes myosin to reach the state of high energy ( excitation), in which myosin head gets detach from the active site.
