The reservoir for the gaseous type of bio-geo chemical cycle exists in
Biogeochemical cycles
The movement of nutrient elements like carbon, nitrogen, sulfur, and phosphorus through components of the ecosystem is called nutrient or biogeochemical cycling. This ensures that valuable nutrients are not lost.Reservoir
The location in the biosphere where the particular nutrient is present at a high concentration and for a long period is called a reservoir in a biogeochemical cycle. For gaseous cycles, the reservoir is usually the atmosphere whereas for sedimentary cycles it is the lithosphere.
Final answer
b. atmosphere