Which of the following metal oxides cannot be reduced by heating with carbon?

As Ca is a reactive metal and is present above hydrogen in the reactivity series, it forms a strong ionic bond with oxygen by transferring two electrons to oxygen.

As a result of strong ionic bonding, CaO cannot be reduced by heating with carbon.

On the other hand, elements like Fe, Zn, and Cu are comparatively less reactive and form weaker bonds with oxygen compared to Ca. Hence their oxides can be easily reduced by heating with carbon.

For example: $\mathrm{ZnO}+\mathrm{C}\to \mathrm{Zn}+\mathrm{CO}$

Hence the correct answer is CaO.