A. Photosynthesis is the process in which plants prepare their own food using carbon dioxide and water which are converted into glucose in the presence of sunlight along with the release of oxygen.
B. Denitrification is a process of reduction of nitrate into molecular nitrogen by denitrifying bacteria.
C. Carbon fixation is the reduction and incorporation of inorganic carbon into organic compounds.
D. Cellular respiration is a metabolic reaction in which energy is released from the breakdown of complex substances. Oxygen is used in respiration and carbon dioxide is released. Hence, carbon is returned to its reservoir through cellular respiration.
So, the correct answer is 'cellular respiration'.