On a global basis, which of the following is the largest reservoir of carbon?

A. Atmosphere

B. Vegetation

C. Sediments

D. Fossil Fuels

From the given choices, the fossil fuels have the most carbon because the fossil fuels such as gasoline, oil, natural gas, and coal. Since the fossil fuels are millions of years old, almost all of its radioactivity had gone down leaving only 14C.

Fossil Fuel ...

because fossil fuel is mainly Hydrocarbon.

And Hydrocarbon is the one which contain Carbon as the primarily element.