Answer:
The correct answer would be A. Reactant.
Reactants are the chemical substances which are being used and transformed in order to produce products in a chemical reaction.
Glucose is used as a reactant in a cellular respiration.
In presence of oxygen, it is fully oxidized to produce energy (adenosine triphosphate or ATP), water and carbon dioxide.
The overall reaction of cellular respiration can be summarized as:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP