Why is $FeO_{(s)}$ not formed in stoichiometric composition?

$NaCl$ is heated in an atmosphere of sodium vapour. The resultant yellow colour is due to the formation of

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ In a particular point defect,an ionic solid is electrically neutral,even if few of its cations are missing from its unit cells.
Reason $(R):$ In an ionic solid,Frenkel defect arises due to dislocation of cation from its lattice site to interstitial site,maintaining overall electrical neutrality.
In the light of the above statements,choose the most appropriate answer from the options given below:

The correct statement$(s)$ regarding defects in solids is(are):
$A$. Frenkel defect is usually favoured by a very small difference in the sizes of cation and anion.
$B$. Frenkel defect is a dislocation defect.
$C$. Trapping of an electron in the lattice leads to the formation of $F$-center.
$D$. Schottky defects have no effect on the physical properties of solids.

Due to $Schottky$ defect:

Which defect leads to an increase in the density of a solid?