<span>Among the choices listed it is the carbon cycle and the nitrogen cycle are the best candidates because they both are involve in the process of photosynthesis. However, I think it is the carbon cycle that involves carbohydrates rather than the nitrogen cycle.</span>
The carbon cycle. Although it's not specific, it talks about combustion or respiration. The reason I chose the carbon cycle, is because nitrogen DOES appear in photosynthesis, but you were strictly talking about carbohydrates.