Question

what was the major contributor of oxygen to earths early atmosphere?
a photosynthesis
b organic respiration
c ozone decomposition
d decomposition of ocean water

Explanation:

Early Earth had little free oxygen. Cyanobacteria, through photosynthesis, began releasing oxygen as a byproduct, which gradually accumulated in the atmosphere. Organic respiration consumes oxygen, ozone decomposition breaks down ozone (a form of oxygen) rather than producing free oxygen for the atmosphere, and ocean water decomposition is not the primary source of early atmospheric oxygen.

Answer:

A. photosynthesis