Which of the following oxides turns yellow on heating and becomes white on cooling?

At room temperature, zinc oxide is a white substance. It becomes yellow when heated because oxygen is lost.

$\mathrm{ZnO}\underset{⟶}{⟶}{\mathrm{Hn}}^{2+}+\frac{1}{2}{\mathrm{O}}_2+2{\mathrm{e}}^{-}$

Because ZnO creates a non-stoichiometric compound at high temperatures, it is white at low temperatures and yellow at high temperatures.

Similarly K₂O also truns yellow on heating while become white at low temperature.

Thus, a and b are correct.