Fatigue during high intense exercise is felt:

The correct option is B due to accumulation of lactic acid
When the ATP supply fails to meet the consumption of ATP during exercise, fatigue occurs. Fatigue during exercise can also be the result of pathologic conditions, including diseases that affect oxygen uptake, energy metabolism, or neuromuscular function. During high intense exercise, their is insufficient supply of oxygen which leads to anaerobic respiration for that time. During anaerobic condition, lactic acid is formed which gets accumlated in the muscle tissues causing fatigue.

Lactic acid is an important fuel source for muscles during exercise, including those in the heart.

So, the corerct answer is "option B"