The correct answer is - A. 780 GtC.
If the inputs and outputs of carbon are as shown on the image, than the carbon levels in the atmosphere will reduce by 20 GtC in a ten year time, thus making the atmosphere have 780 GtC instead of the initial 800 GtC.
On the image we can see that the carbon that gets up into the atmosphere is 69 GtC annually, while the carbon that returns on the Earth is 71 GtC annually. That means a drop of 2 GtC annually in the carbon levels in the atmosphere.
Such a process will reduce the Greenhouse effect, and eventually it will lead to an ice age because the atmosphere will not be able to retain as much heat as previously, thus the Earth will cool off.
