Final answer:
Oxygen is a by-product of photosynthesis as it is produced as a waste product during the process. It is released into the atmosphere while plants use glucose as their food source. The primary topic of this question is the role of oxygen in photosynthesis.
Step-by-step explanation:
Oxygen is considered a by-product of photosynthesis because it is produced as a waste product during the process. During photosynthesis, plants use sunlight, carbon dioxide, and water to produce glucose and oxygen. Glucose is the main product of photosynthesis and acts as a food source for plants, while oxygen is released into the atmosphere.