Reactant is what ever is started off with (aka every thing on the left side of a reaction equation)
Photosynthesis equation is:
6CO2 + 6H2O -------------> C6H12O6 + 6O2
This means that the reactants are:
6CO2 + 6H2O -------------> C6H12O6 + 6O2
This is carbon dioxide (six of them to be specific) and six waters, light energy is also required
Hope this helped!