Why does the heart automatically adjust the flow of blood to match activity levels?
a.
extreme activity damages blood cells and the body needs replacements.
b.
the heart requires more oxygen when exercising.
c.
it builds up a higher level of oxygen in the blood.
d.
organs and muscles require more blood under stress.

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. 

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.

Iwould say a is correct