Which process in the carbon cycle involves the breathing out of CO2?

Respiration is the process in the carbon cycle where organisms, including plants and animals, break down glucose to obtain energy for their cellular activities. During this metabolic process, oxygen is used to facilitate the conversion of glucose into energy, and carbon dioxide is produced as a waste product. This released CO2 is then sent back into the atmosphere.

In contrast, photosynthesis, which occurs primarily in plants, utilizes carbon dioxide and sunlight to create glucose and oxygen, effectively removing CO2 from the atmosphere. Combustion involves burning carbon-containing materials, which also releases CO2, but it is a different process than biological respiration. Nitrification, on the other hand, is a step in the nitrogen cycle and is not directly related to the carbon cycle. Thus, respiration is the correct answer as it specifically addresses the release of CO2 through breathing.