Gibbs energy vs T plot for the formation of oxides is given below....

Ellingham diagrams are a particular graphical form of the principle that the thermodynamic feasibility of a reaction depends on the sign of $\Delta G$, the Gibbs free energy change, which is equal to $\Delta H - T \Delta S$, where $\Delta H$ is the enthalpy change and $\Delta S$ is the entropy change. Hence, element below in Ellingham diagram can reduce oxide of element above it.

$C$ can reduce $FeO$ at $600 º C$.