Electron-deficient impurities are caused by the elements of which group?

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.

The defect observed when $NaCl$ is heated in an atmosphere of sodium vapor is ................. .

Explain the following term with a suitable example:
$(ii)$ Frenkel defect

The formula of nickel oxide is $Ni_{0.98}O_{1.00}$. What is the approximate percentage of $Ni^{2+}$ in it?

Which of the following statements is $WRONG$?