Which of the following processes adds CO_2 to the atmosphere?

Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize nutrients from carbon dioxide and water, thereby removing $\text{CO}_2$ from the atmosphere.

Respiration is a metabolic process in living organisms involving the production of energy, typically with the intake of oxygen and the release of carbon dioxide, thus adding $\text{CO}_2$ to the atmosphere.

Carboxylation is a chemical reaction in which a carboxylic acid group is produced by treating a substrate with carbon dioxide, which consumes $\text{CO}_2$.

Nitrification is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate, which does not release $\text{CO}_2$.

Therefore, respiration is the process that adds $\text{CO}_2$ to the atmosphere.

Answer: Respiration