Why do muscle cells sometimes respire anaerobically during heavy exercise?

The correct option is: Because the supply of oxygen is limited

During heavy or vigorous exercise, the muscles require a large amount of energy (in the form of adenosine triphosphate, or ATP) very quickly to sustain rapid contractions. Under normal conditions, cells rely on aerobic respiration, which uses oxygen to break down glucose and release energy efficiently.

However, during intense physical activity, the cardiovascular and respiratory systems cannot deliver oxygen to the muscle cells fast enough to meet this elevated energy demand. Since the supply of oxygen is limited, the cells temporarily switch to anaerobic respiration (respiration without oxygen) to continue producing energy rapidly, resulting in the production of lactic acid as a byproduct.