Answer: E. Lactic acid
Explanation: Lactic acid is what causes muscle soreness after intense exercise. This is because of a process known as fermentation.
Humans normally require oxygen to undergo cellular respiration. However, if the body senses that there is a lack of oxygen in the body, it will proceed to use lactic acid fermentation rather than the subsequent aerobic steps that take place after glycolysis. During intense exercise, muscles may lack oxygen due to its excessive use, thus leading to lactic acid fermentation taking place.
During lactic acid fermentation, the pyruvate products of glycolysis are converted into lactic acid by NADH molecules adding hydrogen atoms to pyruvate. This is necessary to convert NADH back into NAD+ so that the body can reuse these molecules in glycolysis to produce more ATP. However, as lactic acid builds up in the cell, it can cause feelings of muscle soreness and fatigue.