We continuously breathe oxygen and also the population is rising at an alarming rate. So why is the oxygen not fully used up?

The correct option is C

The plants take carbon dioxide and release oxygen while animals take oxygen and release carbon dioxide.

Plants make their own food in a process called photosynthesis. In this process, oxygen is produced along with food.
Plants also consume oxygen for respiration, but they produce more of it than they consume.
Animals, on the other hand, take in oxygen during respiration and expel out carbon dioxide. Thus the balance of oxygen and carbon dioxide in the atmosphere is maintained. This also shows the interdependence of plants and animals.