Carbon is used as a common reducing agent in the extraction of

Metals Zn, Pb, Fe are placed middle of the reactivity series. Hence, these metals can be reduced by using reducing agent.

$\mathrm{ZnO}+\mathrm{C}\stackrel{∆}{\to }\mathrm{Zn}+\mathrm{CO}; \mathrm{PbO}+\mathrm{CO}\stackrel{∆}{\to }\mathrm{Pb}+{\mathrm{CO}}_2$

${\mathrm{Fe}}_2{\mathrm{O}}_3+3\mathrm{CO}\stackrel{∆}{\to }2\mathrm{Fe}+3{\mathrm{CO}}_2$

Note: Silver, Ag is noble element and it does not react with other elements.