You would need to radioactively label water (H₂O) to produce radioactive oxygen during photosynthesis.

Here's why:

* Photosynthesis is the process where plants use sunlight, water, and carbon dioxide to produce glucose (sugar) and oxygen.

* The oxygen released during photosynthesis comes from the splitting of water molecules. This process is called photolysis.

* Radioactive labeling allows scientists to trace the path of specific atoms within a biological process. By labeling water with a radioactive isotope, they can track the movement of oxygen atoms as they are incorporated into the oxygen molecule.

So, by labeling water with a radioactive isotope (like oxygen-18), you can track the movement of the oxygen atoms and see how they are ultimately released as radioactive oxygen gas during photosynthesis.