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's atmosphere lacked significant oxygen until cyanobacteria (photosynthetic organisms) evolved. Photosynthesis converts carbon dioxide and water into glucose and oxygen, releasing oxygen as a byproduct. This process gradually accumulated oxygen in the atmosphere over millions of years. Organic respiration consumes oxygen, ozone decomposition breaks down ozone (not a major oxygen source), and ocean water decomposition is not a primary contributor to early atmospheric oxygen.

Answer:

A. photosynthesis