Correct Option: A  [ water ]
Remark:
Explanation: During photosynthesis, green plants use sunlight to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂). The oxygen gas released in this process comes specifically from the splitting of water molecules — a reaction known as photolysis of water, which occurs in the light-dependent reaction inside the chloroplasts.
Chemical equation:
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
The oxygen atoms in the released O₂ come entirely from the original water molecules, as demonstrated by isotopic experiments using heavy oxygen (O¹⁸) tracers.