Final answer:
The radioactive 18O given to a plant in the experiment would be used to make sugars via the process of photosynthesis, with water being the correct answer to the question.
Step-by-step explanation:
In the experiment described, a plant was given a substance containing radioactive 18O to trace the formation of sugar. Since the process of photosynthesis is responsible for the synthesis of sugars in plants, and this involves the uptake of carbon dioxide (CO2) and water (H2O), using sunlight to convert these into glucose (a form of sugar) and oxygen, the radioactive Oxygen isotope would be integrated into sugar molecules.
The correct answer to the question is therefore: a) Water. The water containing the radioactive oxygen would be split during the light-dependent reactions of photosynthesis, incorporating the 18O into the sugars being produced, notably glucose, in the plant's chloroplasts, specifically in the mesophyll cells.