Answer;
Living organisms
Step-by-step explanation;
-The process responsible for producing most of the Earth’s oxygen is known as photosynthesis. In this process, plants and certain other organisms convert sunlight into oxygen and other products such as glucose (simple sugars).
-Tiny organisms known as cyanobacteria, or blue-green algae, were microbes that conducted photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and oxygen, thus accounting for the source of oxygen in the atmosphere.