Question

photosynthetic cyanobacteria caused an increase in atmospheric levels of
a ozone
b water
c oxygen
d carbon dioxide.

Explanation:

Photosynthetic organisms, including cyanobacteria, perform photosynthesis which uses carbon dioxide and water, producing glucose and oxygen as a byproduct. Early cyanobacteria were responsible for the Great Oxidation Event, which drastically increased atmospheric oxygen levels. Ozone forms from oxygen, water levels are not primarily increased by cyanobacterial photosynthesis, and carbon dioxide is consumed rather than produced by this process.

Answer:

C. oxygen