During heavy exercise, we get cramps in the legs due to the accumulation of
A. Carbon dioxide
B. Lactic acid
C. Alcohol
D. Water

Hint: One of the end products of fermentation is responsible for cramps in our legs. This compound also takes part in the Cori cycle.

Complete answer
Muscle cramps occur due to the accumulation of lactic acid during the anaerobic breakdown of glucose. It usually occurs as the body needs a quick supply of energy during heavy muscular exercise. The instantaneous energy sources of muscle include ATP and phosphocreatine which get depleted quickly. After that, the glycogen stored in our muscle cells is converted to glucose which is metabolized non oxidatively to form ATP and lactic acid as the end product.
The anaerobic breakdown of glucose has a great advantage as it provides energy very quickly but the ATP yield in this process is very low. Rapid depletion of glycogen stores in muscles occurs and therefore energy supply stops. Further lactic acid accumulates within our muscular tissue which needs to be removed.
This nonoxidative or anaerobic mode of glucose metabolism provides almost 100-fold more energy than what is obtained by using ATP and phosphocreatine stored in muscles. Therefore we can understand that when these short term sources of energy get depleted it results in tiredness or fatigue of muscles. The lactic acid accumulated is responsible for causing muscle cramps.

So, the correct answer is, “ During heavy exercise, we get cramps in the legs due to accumulation of lactic acid.”

Note:
Long term sources of energy include oxidation of glucose, lactate, and fatty acids, but the rate of supply of energy is slower. Aerobic oxidation of glucose provides us with almost 15 times more energy than obtained from non-oxidative metabolism of glucose.

In the case of prolonged exercises, hepatic gluconeogenesis takes priority as the glycogen stores in liver hepatocytes or liver cells get depleted. So during very long exercise periods, lactate is released into the blood circulation by contraction of skeletal muscles. This lactate is taken up by the liver cells and resynthesized into glucose for reuptake by the skeletal muscle cells. This specific cycle where lactate is recycled is known as the Cori Cycle.