This is because Carbon Dioxide is a reactant in the Photosynthesis process. Oxygen is an output as plants release what humans need to breathe. Glucose is also an output as the point of photosynthesis is for plants and autotrophs to create their own food, aka glucose.
Glucose is a sugar that plants create using sunlight, water, and carbon dioxide. Oxygen is a side effect of the plant making glucose for food. The correct answer is Carbon dioxide. Carbon dioxide is used for photosynthesis, not created by it.
