The correct option is C ΔS(C(s)+12O2(g)→CO(g))<ΔS(C(S)+O2(g)→CO2(g))
The line corresponding to the formation of CO2 is nearly horizontal.
Till 710∘C, ΔG∘f for CO2 is more negative.
But after 710∘C, ΔG∘f for CO is more negative
and thus the formation of CO is thermodynamically favoured.
Don't forget - the reactions are all for the formation of one mole of the oxide.
In the formation of the monoxide, 0.5 mol of a O2 yields 1 mol of CO.
Contrast this with the formation of CO2:
One mole of O2 yields one mol of CO2. Hence the answer.