Question

the primitive atmosphere of earth was deficient in free oxygen. what process was primarily responsible for the development of the present percentage of free oxygen in the earths atmosphere?
photosynthesis
volcanic eruptions
outgassing
oxidation of iron - based minerals

Explanation:

Photosynthesis is the process by which plants, algae, and some bacteria convert carbon dioxide and water into glucose and oxygen using sunlight. Over billions of years, this process has significantly increased the amount of free oxygen in the Earth's atmosphere. Volcanic eruptions release gases like water vapor, carbon dioxide, and sulfur dioxide, but not a large amount of free oxygen. Outgassing is related to the release of gases from the Earth's interior, mainly carbon - based gases. The oxidation of iron - based minerals consumes oxygen rather than producing it on a large scale to account for the present - day oxygen levels.

Answer:

photosynthesis