Cyanobacteria or simply known as blue-green algae may have formed the oxygen into the early atmosphere. This is because these micro-organism has the capability to photosynthesize wherein they can produce oxygen and carbohydrates. In fact, all the plants contribute in the fomation of oxygen.
We can also assume that this organism contains membrane bound organelles, and I believe is categorized as a Protist.
All the branches of science that deal with the scientific study of living organism are called life sciences. e.g; botany,zoology,anatomy,biochemistry,embryology etc..
Haiii !! I believe the third option to be reasonable
