Final answer:
During exercise, mitochondrial PO2 remains constant as the body has mechanisms to supply enough oxygen to meet the increased demand.
Step-by-step explanation:
During exercise, mitochondrial PO2 remains constant (c). Intense muscle activity increases the demand for oxygen, but the body has mechanisms in place to supply enough oxygen to the mitochondria. The increased breathing rate after exercise helps to meet the oxygen demand and compensate for the oxygen debt that occurs during intense muscle activity. Therefore, even though there is an increased demand for oxygen during exercise, the body maintains mitochondrial PO2 at a constant level.