Final answer:
The substance necessary for a plant to conduct light reactions in photosynthesis is b)chlorophyll. It absorbs sunlight and initiates the production of glucose and oxygen.
Step-by-step explanation:
The substance a plant must contain to conduct light reactions in photosynthesis is chlorophyll. This vital pigment is present in the chloroplasts of green plants and is responsible for absorbing light energy, which is essential for converting carbon dioxide and water into glucose and oxygen. Chlorophyll specifically captures the light energy that drives the synthesis of glucose through a series of reactions known as the light-dependent reactions of photosynthesis.
During these light-dependent reactions, chlorophyll absorbs sunlight and initiates a chain of events that leads to the production of chemical energy in the form of ATP and NADPH. These products are then used in the light-independent reactions, or the Calvin cycle, to convert carbon dioxide into glucose. Without chlorophyll, a plant cannot start the process of photosynthesis, making it the correct answer to the student's question. So the answer is option b.