Final answer:
The addition of oxygen to the early Earth's atmosphere most likely occurred through the process of photosynthesis.
Step-by-step explanation:
The addition of oxygen to the early Earth's atmosphere most likely occurred through the process of photosynthesis. Photosynthesis is a biological process in which plants use sunlight to convert carbon dioxide and water into oxygen and glucose. Over time, the increasing population of plants and the evolution of more efficient oxygen-utilizing pathways led to the oxygenation of the atmosphere.