Hello!
Let's look at the equation for photosynthesis:
6CO2 + 6H20 + (energy) → C6H12O6 + 6O2
As you can see, plants produce glucose. This immediately eliminates water and carbon dioxide, as they are the reactants in photosynthesis.
During cellular respiration, glucose is "digested" in a way similar to how we digest our food.
Therefore, it is safe to assume that glucose is used by plants as a source of food.
I hope I helped!
