Final answer:
The reactants of the light reactions in photosynthesis are water and sunlight, which are converted into ATP and NADPH; thus, the correct answer is B) water and sunlight.
Step-by-step explanation:
The reactant(s) of the light reactions in photosynthesis are water and sunlight. During the light reactions, water is split to produce oxygen, and in the process, energy from sunlight is captured and converted into chemical energy in the form of ATP and NADPH. These molecules of ATP and NADPH are then used as energy sources in the subsequent Calvin cycle, also known as the light-independent reactions, to produce sugars such as glucose. Therefore, the correct answer to the student's question is B) water and sunlight.
The light-dependent reactions, which occur in the thylakoid membranes of the chloroplast, harness energy from sunlight to produce ATP and NADPH. These energy carriers are essential for the Calvin cycle, where carbon dioxide is fixed into sugars.
It's important to clarify that ATP and NADPH are not reactants but products of the light reactions, and carbon dioxide is a reactant in the Calvin cycle, not the light reactions.