Among the following groups of oxides, the group containing oxides that cannot be reduced by C to give the respective metal is:

$\mathrm{CaO}$, ${\mathrm{K}}_2\mathrm{O}$ are groups of oxides, that cannot be reduced by $\mathrm{C}$ to give the respective metal.

Because calcium and potassium are potent reducing agents, carbon cannot be used to reduce their oxides. Carbon cannot decrease the oxides of highly reactive metals.