Answer:
Oxygen (O2)
Explanation:
This question is describing photosynthesis which uses carbon dioxide and water as reactants in the presence of sunlight to synthesize sugar (glucose) and oxygen. The general equation is as follows:
6CO2 + 6H2O ---> C6H12O6 + 6O2
According to this question, a plant takes in 6CO2 molecules and 6H2O molecules in order to build one C6H12O6 molecule. Using the balanced equation above, the extra element is OXYGEN, which still needs to be released as a gas for the equation to be balanced i.e 6O2 molecules
