Answer:
Calcium ions
Explanation:
The excitation-contraction coupling process is the process by which the electrical activation of muscles (the depolarization of the action potential) leads to the activation of contraction.
An action potential in the skeletal muscles or muscle excitation leads to Ca release from the sarcoplasmic reticulum via calcium channel protein. Released Ca regulates the contraction of the muscle.
Oxygen is used for breathing
Chlorine is used for purifying water
Nitrogen is used for plants
Potassium is used in fertilizer
C is correct i believe i think all the others are wrong but im not entirely sure