FeO(s) → Fe(s) + O₂(g) [ΔG(FeO, Fe)]

C(s) + O₂ (g) → CO (g) [ΔG (C, CO)]

In an Ellingham diagram, the reducing agent reduces the metal oxide only if ∆G is negative. If the difference in ∆G is large and negative then the reduction happens easily. The overall ΔG for the two reactions given above is

ΔG (C, CO) + ΔG (FeO, Fe) = Δ_rG

From the above diagram,at any temperature above point A,

ΔG (C, CO)< ΔG (Fe, FeO)

So, carbon will reduce FeO to Fe and will itself get oxidised to CO because the summation of ΔG is large and negative.