Answer:
Lactic acid
Step-by-step explanation:
Muscle contraction needs energy in the form of ATP which allows the formation of cross-bridge.
The ATP is formed by the process of cellular respiration which takes place in the presence of oxygen but when the amount of oxygen decreases in the muscle, to produce energy the cell perform anaerobic respiration.
The pyruvate formed as a result of the glycolysis and decarboxylation gets accumulated in the muscles as it does not enter the Krebs cycle. The accumulated pyruvate gets converted into the Lactic acid.
Thus, Lactic acid is the correct answer.