Question - The primitive atmosphere of Earth was deficient in free oxygen. What process was primarily responsible for the development

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 Earth’s atmosphere? 

A) outgassing 

B) photosynthesis 

C) volcanic eruptions 

D) oxidation of iron-based minerals

ANSWER:

B) photosynthesis