The sensation of fatigue in the muscles after prolonged strenuous physical…

Answer: Accumulation of lactic acid.

During prolonged, strenuous physical work the muscles need more energy (ATP) than the available oxygen can support. When oxygen reaches the muscle cells fast enough, glucose is broken down by aerobic respiration. But under heavy exertion the demand outpaces the oxygen supply, so the muscle fibres switch to anaerobic respiration (anaerobic glycolysis).

In anaerobic respiration glucose is only partially broken down: glucose to pyruvate, and pyruvate is then converted to lactic acid (lactate) instead of being fully oxidised to carbon dioxide and water. This lactic acid accumulates in the muscle tissue.

In the classical, textbook account this build-up of lactic acid is given as the explanation for the tired, fatigued feeling in the muscles during and right after hard exercise, which is the answer expected here. Once the activity stops and the oxygen supply is restored, the lactate is gradually oxidised or carried to the liver and reconverted, and the tiredness eases.

Note: modern exercise physiology treats muscle fatigue as multifactorial — hydrogen-ion build-up (acidosis), inorganic phosphate and other factors all contribute, and lactate itself can even serve as a fuel — so lactic acid is the classical explanation rather than the sole direct cause. Lactate also returns to normal within about an hour, so it does not explain the muscle soreness that appears a day or two later (that delayed soreness is linked instead to microscopic muscle damage and inflammation).

Among the options given, accumulation of lactic acid is therefore the correct choice.