The correct option is D Digestion or breakdown of organic compounds
Carbon
is a component of all organic compounds of protoplasm like carbohydrates,
lipids, proteins, etc. It constitutes 49% of dry weight of organisms and is
next only to water in abundance. Carbon cycling occurs through atmosphere,
ocean and through living and dead organisms. Bacteria also play a very
important role in carbon cycle. Decomposers contribute substantially to CO2
pool by their processing of waste materials and dead organic matter of land or
oceans. The main role of bacteria in the carbon cycle involves digestion or
breakdown of these waste organic compounds.