During photosynthesis, plants convert carbon dioxide and water into glucose (C₆H₁₂O₆) and oxygen (O₂) using light energy. The general reaction can be written as:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Radioactive hydrogen (usually tritium, ³H, or deuterium, ²H) used to make water would be incorporated into the water molecules (H₂O). When plants undergo photosynthesis, the hydrogen from the water molecules becomes part of the glucose molecule.
So, the correct statement regarding what happened to the radioactive hydrogen during photosynthesis is:
It was recycled when glucose was formed.