.Out of C and CO which is a better reducing agent at 673 K?

CO acts as a better reducing agent between 500 – 800 K and C acts as a better reducing agent between 900 – 1500 K.
This can be answered on the basis of the Ellingam diagram.
It can be observed from the given graph that above 1073 K (approx.), delta g(C,CO)<delta g(Fe,FeO). So, coke will reduce FeO, and will itself be oxidised to CO.