Answer: Fast
Explanation: Fast carbon cycle include the process which can circulate the carbon in the ecosystem in comparatively less time. For example, carbon dioxide is taken up by plants and converted into food, which is consumed by animals. Animals then release carbon dioxide which is again consumed by plants. So this is a repeatedly undergoing cycle. The main process involved behind this is photosynthesis. All the three- plants, algae and phytoplanktons perform photosynthesis, so they are the part of fast carbon cycle.