In the process of photosynthesis, radioactive hydrogen (specifically in the form of radioactive water, H₂O) is incorporated into the plant's biological processes. The correct answer to your question about what happens to the radioactive hydrogen during photosynthesis is:
D: it was recycled when glucose was formed.
During photosynthesis, plants convert carbon dioxide and water into glucose and oxygen using light energy. The hydrogen from the radioactive water is incorporated into glucose molecules during this process, and as glucose is formed, the radioactive hydrogen is effectively "recycled" into the plant's biochemical pathways.