Final answer:
In photosynthesis, water is a reactant. It is used along with CO2 to produce glucose and oxygen via the process of photosynthesis. The correct answer to the student's question is A. Reactant.
Step-by-step explanation:
The term that best describes water in the context of photosynthesis is reactant. In photosynthesis, water (H2O), along with carbon dioxide (CO2), are the primary reactants that, under the influence of sunlight and chlorophyll, are converted into glucose (C6H12O6) and oxygen (O2) which is considered a waste product because it is not used in the reaction but is released into the atmosphere. Therefore, the correct answer to the student's question is A. Reactant.
To elaborate further, cellular respiration and photosynthesis are two critical processes in biology which exhibit a form of biochemical reciprocity. Cellular respiration is an exergonic reaction where glucose is broken down with the consumption of oxygen to produce carbon dioxide, water (as a product), and ATP - the energy currency for cells. Conversely, photosynthesis is an endergonic reaction that requires energy input from light to convert water and carbon dioxide into glucose and oxygen. In both processes, water plays a pivotal role - as a product in cellular respiration and as a vital reactant in photosynthesis.