Which one of the following has Frenkel defect?

Which statement is correct regarding the $F$-center?

Potassium crystallizes in an $FCC$ lattice with a unit cell edge length of $0.5 \ nm$. The approximate density (in $g \ cm^{-3}$),if it contains $0.1 \%$ Schottky defects,is:

If an electron is present in place of an anion in a crystal lattice,then it is called

In spite of long-range order in the arrangement of particles,why are the crystals usually not perfect?

What type of crystal defect is indicated in the diagram below?
$Na^{+} \, Cl^{-} \, Na^{+} \, Cl^{-} \, Na^{+} \, Cl^{-}$
$Cl^{-} \, \Box \, Cl^{-} \, Na^{+} \, \Box \, Na^{+}$
$Na^{+} \, Cl^{-} \, \Box \, Cl^{-} \, Na^{+} \, Cl^{-}$
$Cl^{-} \, Na^{+} \, Cl^{-} \, Na^{+} \, \Box \, Na^{+}$