Questions: The radioactive isotope used in the Calvin experiments was carbon-12 nitrogen-15 carbon-14 boron-8

The answer is the third one: carbon-14.

Explanation for each option:

• Carbon-12: This is the most common isotope of carbon and is stable, not radioactive. It was not used in the Calvin experiments for tracing purposes.

• Nitrogen-15: This is a stable isotope of nitrogen and is not radioactive. It was not used in the Calvin experiments.

• Carbon-14: This is a radioactive isotope of carbon and was used in the Calvin experiments. The Calvin cycle experiments, conducted by Melvin Calvin and his colleagues, used carbon-14 to trace the path of carbon in photosynthesis.

• Boron-8: This is a radioactive isotope, but it is not related to the Calvin experiments. It was not used in these experiments.

In summary, carbon-14 was the radioactive isotope used in the Calvin experiments to trace the incorporation of carbon into organic compounds during photosynthesis.