Which of the following compounds exhibits metal deficiency defect?

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:

In a crystal,Frenkel defect is observed only when the radius ratio is......

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 $g/cm^3$ $(N_A = 6 \times 10^{23})$.

The flame colours of metal ions are due to

$1 \ \text{mole}$ of $NaCl$ is doped with $10^{-5} \ \text{mole}$ of $SrCl_{2}$. The number of cationic vacancies in the crystal lattice will be: