The sensation of fatigue in muscles after prolonged strenuous physical work is primarily caused by (d) Accumulation of lactic acid. During intense exercise, when oxygen demand exceeds supply, the body resorts to anaerobic metabolism, breaking down glucose without oxygen. This process produces lactic acid as a byproduct, leading to an increase in acidity within the muscles.

The buildup of lactic acid contributes to muscle fatigue and discomfort, resulting in a burning sensation and reduced muscle function. However, it’s important to note that fatigue can also be influenced by other factors such as depletion of glycogen stores, minor muscle fiber damage, and central nervous system fatigue.

While decreased oxygen supply (a) can contribute to fatigue in prolonged exercise, it’s primarily the buildup of lactic acid that causes the acute sensation of muscle fatigue during intense physical activity.