Which organism is most likely responsible for first producing oxygen in Earth's early atmosphere? How did the work of J. William Schopf help
demonstrate this theory? Reference evidence from the text to support
your answer.

A organism is most likely responsible for first producing oxygen in  Earth's early atmosphere is explained below in details.

Explanation:

The answer is little organisms identified as cyanobacteria or blue-green algae. These microorganisms carry photosynthesis: practicing sunshine, water, and carbon dioxide to compose carbohydrates and, yes, oxygen. Earth's oxygen but the largest (70%) of the oxygen in the environment is generated by oceanic plants. The ocean creates oxygen through the plants that exist in it.

i am positive it is c. condensing but i may be wrong

i think the answer is a.keratin is the pigment responsible for skin color.