The correct answer is (b).
Muscle fatigue occurs due to the accumulation of lactic acid. Lactic acid is a byproduct of glycolysis, which is the process by which the body breaks down glucose for energy. When the body is working hard, it produces more lactic acid than it can use. This excess lactic acid builds up in the muscles, causing them to become sore and tired.
Pyruvic acid is another byproduct of glycolysis. However, it is not directly responsible for muscle fatigue. Pyruvic acid can be converted into lactic acid, but it can also be converted into other substances that the body can use for energy.
Oxaloacetic acid and malic acid are intermediates in the Krebs cycle, which is the second stage of cellular respiration. The Krebs cycle produces energy in the form of ATP. However, it does not produce lactic acid.
In conclusion, muscle fatigue occurs due to the accumulation of lactic acid. Lactic acid is a byproduct of glycolysis, which is the process by which the body breaks down glucose for energy. When the body is working hard, it produces more lactic acid than it can use. This excess lactic acid builds up in the muscles, causing them to become sore and tired.