When you exercise, you use more oxygen to fuel your cells, so your body can function at a faster pace. Oxygen is what your cells need, carbon dioxide isn't. Carbon dioxide is a waste we breathe out through the respiratory process.
To pump more blood to the body, the heart would need to pump faster.
So, the best answer is "C". We need oxygen, and your heart rate pumps faster.
I hope this helps!
~kaikers