The right answer is A) Planting a field of wheat.
The carbon cycle depends on the climate and vice versa. Interactions are complex, with phenomena that contribute to positive feedback, others to negative feedback. Not all phenomena are fully identified and quantified, but we can still cite a few:
* Atmospheric CO2 and methane contribute to the greenhouse effect: an increase in their rate is a factor of global warming.
* Global warming reduces the dissolution of CO2 in the oceans.
* Global warming can reduce the transfer of CO2 to deep waters, and thus further reduce its dissolution in surface waters.
* Global warming increases the degradation of organic matter in frozen soils (permafrost) and peaty environments, which releases CO2 and methane, exacerbating the additional greenhouse effect.