Final answer:
Water is a reactant in photosynthesis, participating in light-dependent reactions and contributing to the formation of oxygen and energy-rich compounds necessary for producing glucose.
Step-by-step explanation:
The role of water in photosynthesis is crucial, as it is a reactant in the process. During photosynthesis, water molecules are split into oxygen, protons, and electrons through a series of reactions known as the light-dependent reactions. These electrons and protons are then used in further reactions to produce energy-rich compounds like ATP and NADPH, which are required for the Calvin cycle (light-independent reactions) to fix carbon dioxide (CO2) into organic molecules such as glucose. Therefore, the correct answer to the question is B) Water is a reactant in photosynthesis. The overall chemical equation for photosynthesis is 6CO2 + 6H2O + energy (sunlight) → C6H12O6 + 6O2, clearly showing water as a reactant and not a product, catalyst, or irrelevant entity.