A white solid which is yellow when hot but changes to white again on cooling is _______
A.$PbO$
B.$CaO$
C.$A{g_2}O$
D.$ZnO$

Hint: The knowledge on stoichiometric affect will help you to find the solution. Also knowing the reactivity of the given metals while heating will help you easily crack the code of the problem. We have to go through each compound to analyse the product obtained in the reaction mentioned in the question.

Complete step by step answer:
Let us first know what the Stoichiometric effect is. It is nothing but the balancing of equations quantitatively so that we may understand the reaction of certain elements during chemical reactions. Sorting them and balancing them will give us an idea of how they react (like the colour changing given in the question). Let us know how every single compound of the question will react to heating.
The first one is the Lead Oxide ($PbO$), which is pale yellowish in appearance, on heating it will give Lead $\,(IV)\,$ Oxide, which is red in colour, the “red lead”. So definitely it is not the answer.
The second one is the Calcium Oxide or Oxocalcium, commonly known as quicklime or burnt lime, white in color. On heating it, it won’t decompose to give a new compound or color, rather it glows on very high temperatures. It is a very stable compound in which the bonds between highly electronegative $O$ and highly electropositive $Ca$ are very strongly bound. Hence, $CaO$ is not the correct answer.
$A{g_2}O$, the Silver Oxide, is a stable oxide of Silver, which is fine black or brown black in color, which violates the question. But let us know what it does when it is heated. It undergoes decomposition reaction yielding silver (in solid form) and Oxygen (in gaseous form). Which is ‘silverish’ in color.
The last one, the Zinc Oxide ($ZnO$), is a white colored compound, which on heating the Zinc ($Z{n^{2 + }}$) ions get detached from their original places and get placed in the interstitial places. This defect is known as Metal-excess defect. Compounds with such defects generally are colored. Hence when heating the ions get replaced and when back to room temperature.
Hence the answer for the given question is Option D: $ZnO$



Note:
The important points to be made into consideration while trying to solve questions of similar types is to look at the behaviour of the cations and anions of such products. A single metal can form three different kinds of Oxides with oxygen. Every single oxide will have different characters and behaviors. In order to solve such questions, go through the electronegativities, number of electrons shared, behavior of cation and behaviour of anion.