Correct Answer: Lactic acid

Solution : The correct option is Lactic acid.

Lactic acid (lactate) is one of the primary compounds responsible for muscular  fatigue. During vigorous or sustained muscular exercise, oxygen delivery may fall short of meeting energy needs, resulting in anaerobic metabolism. This causes lactic acid to build up in the muscles, contributing to tiredness, muscular discomfort, and a decline in muscle function.