Muscle fatigue occurs when the muscles are no longer able to perform at their optimal level. It is typically caused by a combination of factors, including depletion of energy stores, inadequate oxygen supply, and the accumulation of waste products. The correct answer provided is option 2: Lactic acid.

During intense exercise or activity, the muscles require energy to contract and perform their function. This energy is primarily obtained from the breakdown of glucose through a process called glycolysis. However, when the demand for energy exceeds the supply of oxygen, as is often the case with high-intensity exercise, an alternative pathway called anaerobic glycolysis takes over.

During anaerobic glycolysis, glucose is broken down into a compound called pyruvate, which is further converted into lactic acid. This process occurs in the absence of oxygen and is responsible for the accumulation of lactic acid in the muscles. The build-up of lactic acid can lead to a decrease in muscle pH, disrupting normal muscle function and contributing to the sensation of fatigue.

It is important to note that while lactic acid has been traditionally blamed for muscle fatigue, newer research suggests that it may not be the sole cause. Other factors, such as potassium imbalance, muscle damage, and central nervous system fatigue