The correct answer is: A) lactic acid.
During the exercise, muscle cells use anaerobic respiration in order to make enough energy in the form of ATP. Anaerobic respiration doesn’t require oxygen, but glucose is broken down to produce energy. Glucose is broken down to pyruvate in the process called glycolysis. Next step is convert of pyruvate into lactic acid (or lactate).
During the exercise, the oxygen level in muscle cells decreases, which results in anaerobic respiration of the cells.
