When you exercise, you burn as fuel.
endorphins
calories
oxygen
water (H20)

Calories is the answer;

Endorphins are the chemicals released when you are exercising, they normally make you happier.

Oxygen is what we use to breath, we don't burn it for fuel.

Water is what we drink, and is what our bodies use to produce sweat while exercising.

This leaves with your answer, which is calories.

I hoped this helped you out.

                                                        water

a colorless, transparent, odorless, tasteless liquid that forms the seas, lakes, rivers, and rain and is the basis of the fluids of living organisms.

                                                        halon  

any of a number of unreactive gaseous compounds of carbon with bromine and other halogens, used in fire extinguishers, but now known to damage the ozone layer.

                                                  dry chemical

extinguishing agent used on class a, class b, and class c fires.

                                                  carbon dioxide

a colorless, odorless gas produced by burning carbon and organic compounds and by respiration. it is naturally present in air (about 0.03 percent) and is absorbed by plants in photosynthesis.