(c) Lactic acid
During vigorous exercise, anaerobic respiration occurs in muscle cells, converting pyruvate to lactic acid. Its accumulation causes painful muscle cramps.
The key concept is anaerobic respiration in animals: when oxygen is insufficient (vigorous exercise), pyruvate → lactic acid (not ethanol, which occurs in yeast/plants). Lactose is a sugar, not a respiration product. Glucose is the starting material, not a product of pyruvate breakdown.