Which type of defect is observed in crystals like $NaCl$ and $KCl$?

Zinc oxide (white) is heated to high temperature for some time. Observe the following statements regarding the above process:
$I$. Zinc oxide colour changes to pale yellow
$II$. The type of defect formed is 'metal deficiency'
$III$. Some $Zn^{2+}$ and $e^{-}$ are present in interstitial places
The correct statements are:

Ionic solids,which have anionic vacancies due to metal excess defect,develop colour. Explain with the help of a suitable example.

Which of the following defects is observed in steel?

Which of the following compounds exhibits metal deficiency defect?

An element $X$ (atomic weight = $24$) crystallises in a face-centered cubic $(FCC)$ lattice. The edge length of the unit cell is $400 \, pm$. If this lattice contains $4\%$ Schottky defects,calculate the observed density of this element in $g/cm^3$ $(N_A = 6 \times 10^{23})$.