The correct answer is C. ATP.
ATP, or adenosine triphosphate, is the energy currency of the cell. It is used to power all cellular processes, including muscle contraction. When ATP levels are low, muscles become tired and fatigued.
Lactic acid is a byproduct of anaerobic respiration, which occurs when there is not enough oxygen available to produce ATP. Lactic acid can build up in muscles during strenuous exercise, causing them to feel sore and tired. However, lactic acid is not the cause of muscle fatigue.
Na+ ions are important for muscle contraction, but they are not the primary source of energy for muscles.
Sulphates are not involved in muscle contraction.
In conclusion, muscles get tired when there is a shortfall of ATP. ATP is the energy currency of the cell, and it is used to power all cellular processes, including muscle contraction. When ATP levels are low, muscles become tired and fatigued.