The methodology, which has been most useful for investigating the Calvin cycle, is

The methodology, which has been most useful for investigating the Calvin cycle, is radioactive isotope technique.

To study and trace the movement of carbon atoms during the Calvin cycle, researchers have utilized radioactive isotopes of carbon, particularly carbon-14 (¹⁴C). Radioactive isotopes have the same chemical properties as their stable counterparts but emit radiation, making it possible to track their movements within biochemical pathways.