Your body produces heat constantly. Where does this energy come from?

Boiler got it right, but if this is multiple choice, then the answer is c) from the bonds in your food. Taken at a simple level the chemical equation for the respiration of glucose is C6H12O6 + 6O2 -> 6H2O + 6CO2 the same as burning it in air (you can set fire to sugar and fats quite easily, try sprinkling a small pinch of custard powder onto a flame). The body does it in a more complicated way, with a lot more intermediate steps involved, but the net result is the same. The potential chemical energy in food is released by oxidisation to provide energy which is used to create 'energy molecules' that cells use to do work. Since no process to do work is 100% efficient, some of the transfer of chemical energy to work is lost as kinteic energy (heat) which warms us up.

Chemical bonds of your food. It could be oxygen too but it really doesn't produce much energy for us, we need ATP which is made out of Phosphate and other stuff. We usually get that from food just like plants get it from sunlight during photosynthesis.