Question

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

The burning of fossil fuels results in the emission of ${ CO }_{ 2 }$ in the atmosphere.
Photosynthesis is a procedure that changes over ${ CO }_{ 2 }$ into a natural mixture and starch (sugar) utilizing vitality from daylight. ${ O }_{ 2 }$ is additionally discharged during this procedure. Oxygen is taken by human beings and animals and they release carbon dioxide in the air. This ${ CO }_{ 2 }$ is taken up by plants for photosynthesis and thus they release ${ O }_{ 2 }$ in the atmosphere. In this way, oxygen is taken and returned back.
The possibility to make a mistake is that you may choose options A and B. But in the burning of fossil fuels ${ CO }_{ 2 }$ gas is emitted in the atmosphere and in respiration water is formed. So, in these, there is no oxygen gas emitted in the atmosphere.