In this reaction, C acts as a/an: 

$\mathrm{ZnO}+\mathrm{C}\to \mathrm{Zn}+\mathrm{CO}$ 

"Reduction" is the process of obtaining electrons and gaining hydrogen, while "oxidation" is the process of losing electrons and receiving oxygen. Dehydration is the removal of water molecules, whereas hydration is the addition of water.

A reducing agent is one that loses electrons or reduces another chemical entity, whereas an oxidizing agent oxidizes another chemical entity to become reduced itself. In the aforementioned reaction, zinc oxide's (+2) oxidation state balances out oxygen's (2) oxidation state. Any element has a zero oxidation number in its native state. As a result, zinc and carbon both have a 0 oxidation number. Therefore, in the reaction, zinc's oxidation number decreases from (+2) to zero, indicating that zinc has been reduced. In contrast, carbon monoxide's oxidation number has grown from 0 to (+2), indicating that carbon has been oxidized.

Thus C is reducing Zn in this reaction. C act as a reducing agent.

Hence, option C is correct.