Which of the following could be a hypothesis?

Only A and B have nitrogen in them. However, none of the radioactive nitrogen isotopes has a very long half life (ten minutes for the longest, seconds for the others), so I would not expect to see any of them in biological systems. It would require some very sensitive m spectroscopy to even have a chance at detecting radioactive nitrogen contamination. Given the short half lives of the isotopes, it would be much better to look at the soil directly to see if there is constant contamination with nitrogen 16 from nuclear reactors.