In an Ellingham diagram for oxides, a metal/element will reduce the oxide of a different element/metal above. This is purely based on thermodynamic feasibility. When a line shows an abrupt shift in slope, it undergoes a phase transformation. For carbon to be the reducing agent, the carbon line should be below. And for the metal oxide to be reduced in only the gaseous state, the metal to metal oxide line should have had two abrupt changes in slope before it intersects and rises above the Carbon to carbon monoxide line. Hence the answer.