In the process of photosynthesis, carbon dioxide (CO2) and water (H2O) are converted into glucose and oxygen (O2) using sunlight. Given your question about which substance is Y, it depends on where "Y" is located in the diagram you mentioned.
- If Y is a reactant of photosynthesis, it would be H2O or CO2.
- If Y is a product of photosynthesis, it would be O2 or glucose.
Please provide more context or details about the diagram to identify substance Y accurately. However, if it's referring to a typical reactant in photosynthesis, the answer is likely d) H2O or a) CO2.