The correct answer is A. Lactic acid.
Lactic acid is a byproduct of anaerobic respiration, which is a type of respiration that occurs when there is not enough oxygen available. When muscles are working hard, they need more oxygen than they can get from the blood. This causes them to switch to anaerobic respiration, which produces lactic acid as a waste product.
Lactic acid can build up in the muscles, causing them to become sore and cramped. This is why people often get cramps after strenuous exercise.
Ethanol is a type of alcohol that is found in alcoholic beverages. It is not produced by the body during anaerobic respiration.
Pyruvic acid is a chemical that is produced during glycolysis, which is the first step in cellular respiration. It is not a waste product, and it does not cause cramps.
Glucose is a type of sugar that is the body’s main source of energy. It is not produced by the body during anaerobic respiration, and it does not cause cramps.