Final answer:
The carbon cycle mainly involves an exchange of carbon between the geosphere and atmosphere, as it encompasses processes that connect land, water, and air including human contributions through burning fossil fuels.
Step-by-step explanation:
The carbon cycle involves an exchange of carbon between many spheres, but the best answer from the choices provided is a. Geosphere and atmosphere. This is because the carbon cycle includes the movement of carbon through land (geosphere), water (hydrosphere), and air (atmosphere), encompassing processes like respiration, photosynthesis, and sedimentation. Carbon reservoirs such as the atmosphere, oceans, and soil play crucial roles in this cycle. Moreover, human activities like the burning of fossil fuels contribute to the carbon cycle by releasing significant amounts of carbon dioxide into the atmosphere.