Oxygen is returned to the atmosphere mainly by:A.Burning of fossil fuelsB.RespirationC.PhotosynthesisD.Fungi

Oxygen is primarily returned to the atmosphere through the process of photosynthesis. So, the correct answer is:

C. Photosynthesis

Photosynthesis is the biological process in which plants, algae, and some bacteria convert carbon dioxide and water into glucose (a type of sugar) and oxygen using sunlight as an energy source. During this process, oxygen is released into the atmosphere as a byproduct. While respiration by animals and burning of fossil fuels do consume oxygen, they do not return it to the atmosphere in significant amounts as photosynthesis does. Fungi are not directly involved in oxygen production or consumption at a significant scale.