Answer: B. carbon cycle
Explanation: The carbon cycle is the way carbon atoms are reused as the law of conservation tells you mass is neither created nor destroyed and that same thing goes with atoms. You see atoms cycle among chemical compounds and carbon goes from the atmopshere to plants and other living and nonliving things. Now a few things carbon becomes in its cycle are carbon dioxide, carbonic acid, bicarbonate, methane, glucose and calcium carbonate. And trust me there is still more that carbon can become but in the end the basic cycle is just photosynthesis, decomposition, respiration, and combustion.