Which defect causes a decrease in the density of a crystal?

Schottky defect in a crystal arises due to

Which type of crystal defect is observed in the diagram given below?
$Na^{+} Cl^{-} Na^{+} Cl^{-} Na^{+} Cl^{-}$
$Cl^{-} \square Cl^{-} Na^{+} \square Na^{+}$
$Na^{+} Cl^{-} \square Cl^{-} Na^{+} Cl^{-}$
$Cl^{-} Na^{+} Cl^{-} Na^{+} \square Na^{+}$

Which of the following represents the lattice structure of $A_{0.95} O$ containing $A^{2+}$,$A^{3+}$,and $O^{2-}$ ions?

Which of the following statements is $NOT$ true about Schottky defect?

The flame colours of metal ions are due to