Final answer:
The two main sources of oxygen in the air we breathe are green plants and phytoplankton, which produce oxygen through the process of photosynthesis. Cyanobacteria, a type of phototrophic organism, were among the first to oxygenate the Earth's atmosphere.
Step-by-step explanation:
Among the options given, the two main sources of oxygen in the air we breathe are green plants and phytoplankton (option b). Green plants perform photosynthesis, a process during which they convert carbon dioxide (CO2) and water (H2O) into oxygen (O2) and sugars/carbohydrates using sunlight. Not only terrestrial plants contribute to this, but also phytoplankton, which are microscopic plant-like organisms in the ocean. These tiny marine organisms are tremendously important as they provide more than half of the photosynthesis of the open ocean, thereby significantly contributing to the oxygen content of the Earth's atmosphere.
The first organisms that oxygenated the atmosphere were cyanobacteria, which are phototrophic organisms. These cyanobacteria evolved from anaerobic lifeforms that existed before Earth's atmosphere contained molecular oxygen. As more phototrophic organisms appeared, the oxygen content in the atmosphere increased, ultimately allowing for the evolution of complex life that depends on oxygen for cellular respiration.