Final answer:
Oxygen is produced during the light-dependent reactions of photosynthesis through the splitting of water, a process occurring in the chloroplasts that contributes to the oxygenation of Earth's atmosphere.
Step-by-step explanation:
Oxygen is produced during the light-dependent reactions of photosynthesis, specifically when water (H2O) is split to supply electrons to the photosynthetic electron transport chain. This occurs in the chloroplasts of plant cells, where sunlight, absorbed by chlorophyll, initiates a series of redox reactions. These reactions not only produce oxygen as a by-product but also the energy carriers ATP and NADPH, which are then used in the Calvin cycle to produce glucose. Through this process, photosynthesis contributes to the oxygenation of Earth's atmosphere.