Oxygen was the most important gas contributed by stromatolites. Thus, the correct option is D.
What are Stromatolites?
Stromatolites may be defined as rock or microbial reefs that are formed by cyanobacteria (blue-green algae).
Stromatolites are responsible for the oxygenation of the atmosphere. Blue-green algae fed off carbon dioxide and convert it into the marine sediments known as Stromatolites.
Therefore, the correct option for this question is D.
To learn more about Stromatolites, refer to the link:
https://brainly.com/question/1001123
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