The correct option is D Breakdown of organic compounds
The main role of bacteria in carbon cycle involves breakdown of organic compounds. Some cyanobacteria are involved in photosynthesis too, but photosynthesis is primarily carried out by plants. And some proteobacteria are chemoautotrophs, that synthesize organic compounds from basic elemental carbon. But without decomposition of dead organic matter, the carbon in living organisms are not released back into the atmosphere, and will not enter the cycle again.