When carbon acts as a reducing agent, it is either converted into COor CO2 or both.
2C+ O2 ––––––▶2CO
C+ O2 ––––––▶CO2
CO is oxidised to CO2 when it is used as a reducing agent.
2CO+ O2 ––––––▶2CO2
From the Ellingham diagram (refer answer number 3), it is clear thatat the temperature 673 K, the ΔG° of the formation of CO2 from CO ismore negative than the formation of CO or CO2 from carbon. Hence, attemperature 673 K, CO is a better reducing agent than C.