Final answer:
In photosynthesis, oxygen is derived from the reactant water (H₂O). Water molecules are split, and oxygen is released as a byproduct.
Step-by-step explanation:
In photosynthesis, oxygen is derived from the reactant water (H₂O). During photosynthesis, water molecules are split, and oxygen is released as a byproduct. The overall reaction of photosynthesis can be represented as:
6CO₂ + 6H₂O + Energy → C₆H₁₂O₆ + 6O₂
Thus, oxygen is a product of photosynthesis, not a reactant. The other options mentioned, such as CO₂, ATP, glucose, and H₂Se, are not involved in the production of oxygen.