C is the used reducing agent for ZnO and not CO. Let us understand why it is so.

Reduction of ZnO to Zn is carried out at a temperature of 1673 K. However, it has been observed that above 1273 K, the Gibbs free energy of formation of CO₂ from C is lesser than the Gibbs free energy of formation of ZnO. Therefore, C can easily reduce ZnO to Zn under these conditions. 

Also, the Gibbs free energy of formation of CO₂ from CO is always more than the Gibbs free energy of formation of ZnO. Therefore, CO cannot  be used reduce ZnO.

Thus, C is a better reducing agent for reducing ZnO when compared to CO.