Concept:Carbon is removed from the air mainly as carbon dioxide used by plants.Explanation:During photosynthesis, green plants take in carbon dioxide (CO2) from the atmosphere.They use this carbon to manufacture glucose and other food substances.Combustion, respiration, and putrefaction all release carbon dioxide back into the air.Therefore, photosynthesis is the process that reduces the amount of carbon in the air.Answer:D. photosynthesis