Question

Carbon dioxide is returned to the atmosphere primarily by the

Answer 1

Carbon dioxide is returned to the atmosphere primarily by respiration of humans, (exhalation to be precise). The return of carbon dioxide to the atmosphere is also possible by the decay of plants and animals and combustion, which is the process of burning something.

Answer 2

Carbon dioxide is returned to the atmosphere primarily by the Respiration of animals and plants (the consumers). 

Explanation;
Respiration by both plants and animals is an important natural source of carbon dioxide. Carbon dioxide is a by product of the process of respiration, a chemical reaction that plants and animals use to produce energy they need to carry out day to day activities.
Plants and animals use this process to produce energy which is used to fuel normal activities such as growth and movement. The process uses oxygen to break down nutrients and release energy, but also creates water and carbon dioxide as byproducts.