D. Organs and muscles require more blood under stress.
A is incorrect because blood cells don't get damaged due to activity. B is incorrect because it is not the heart that needs oxygen; it's the body cells that are being worked, such as muscle cells in you're arm when you're lifting weights. C is incorrect because the flow of blood doesn't change the level of oxygen in the blood; breathing does.
D. Organs and muscles require more blood under stretch. Just think about it, when you go on a run or do something physical your blood levels increase. Exercising is all about getting your pulse up, because that's how you measure intensity.
