Question

which is the greatest carbon reservoir?
a. geosphere
b. hydrosphere
c. atmosphere
d. biosphere

Explanation:

The geosphere (including rocks, fossil fuels, etc.) stores a vast amount of carbon, much more than the hydrosphere (water - related carbon), atmosphere (gaseous carbon), or biosphere (living organisms' carbon). So the geosphere is the greatest carbon reservoir.

Answer:

A. Geosphere