Oxygen is returned to the atmosphere mainly by (c) photosynthesis.
Explanation: Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to produce oxygen as a byproduct, which is then released into the atmosphere.
Why other options are incorrect:
(a) Burning of fossil fuels:
Burning fossil fuels like coal, oil, and natural gas primarily releases carbon dioxide (CO2) into the atmosphere, not oxygen.
(b) Respiration:
Respiration is the process by which organisms use oxygen to break down glucose and generate energy, resulting in the release of carbon dioxide, not oxygen.
(d) Fungi:
Fungi do not produce oxygen through photosynthesis; they are decomposers that break down organic matter.