Question

Through which process is oxygen returned to the atmosphere?

Answer 1

Oxygen is essential for respiration, the energy-producing chemistry that drives most living things' metabolisms. We humans, like many other creatures, require oxygen in the air we breathe to survive. Plants and many types of microbes produce oxygen during photosynthesis.

Complete answer:
The oxygen cycle refers to the movement of oxygen in various forms throughout nature. When it comes to uncombined elements in the atmosphere, oxygen is second only to nitrogen in terms of abundance. Plants and animals utilise oxygen to breathe and then release carbon dioxide into the air and water.
Carbon Dioxide is then absorbed by algae and terrestrial green plants and transformed into carbohydrates during the photosynthesis process, with oxygen as a by-product. The world's seas are the biosphere's primary oxygen generators; their algae are believed to renew almost $90\%$ of all oxygen used.
All of the other biogeochemical cycles involve oxygen to some extent. Detritus from living organisms, for example, transfers oxygen-containing compounds such as calcium carbonates into the lithosphere over time.
Photosynthesis recycles oxygen back into the atmosphere.
Photosynthesis is the process by which green plants and a few other organisms prepare their food by utilising sunlight, carbon dioxide, and water.
The photosynthetic process converts light energy into chemical energy in plants, algae, and some microbes.
Photosynthesis generates glucose, which offers two key resources to organisms: energy and fixed carbon.

Note:
Despite the use of fossil fuels and the reduction of natural vegetation (on land and at sea), the level of atmospheric oxygen appears to be relatively stable due to increased plant productivity as a result of global agricultural advances.