Solid State Chemistry - Point Defects and Interstitial Compounds

$NaCl$ shows the metal excess defect, which results in the formation of an F-center in the crystal. When light falls on this crystal, the electron in F-center gets excited. This F-center is then responsible for the yellow $(X)$ color of $NaCl$.

$F$-centres or colour centres are points in a crystalline compound at which negative ion missing is replaced by one or more unpaired electrons. 

The statement regarding VO, VO2, VO3, and TiO3 is incorrect because these oxides show metallic or insulating properties depending on the temperature, rather than metallic properties at all temperatures. The other statements correctly describe F-centres, magnetic properties, and close-packing structures.

$F-$ centre is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more unpaired electron, depending on the charge of the missing ion in the crystal. These unpaired electrons produces colour in the crystal by absorbing light in the visible spectrum. The unpaired electron acts as charge carriers and hence the crystal conducts electricity.

When NaCl crystals are heated in sodium vapor, sodium atoms deposit on the surface and chloride ions diffuse to the surface to form NaCl. The remaining electrons occupy the anionic vacancies, creating F-centres, which absorb light and impart a yellow color.

F-centres contain unpaired electrons, which makes the crystal paramagnetic, not diamagnetic. The other statements are correct descriptions of F-centres.

Generally, the ion exchange tendency of a material depends on the extent of isomorphous substitution in the tetrahedral framework. Thus, the $Si^{4+}$ ions of feldspar and zeolite are replaced by $Al^{3+}$ (aluminium ion).

$ZnO$ shows yellow colour of heating due to F-centre

Interstitial compounds are formed when small atoms like hydrogen, carbon, or nitrogen are trapped in the crystal lattices of transition metals. Titanium hydride (TiH) is a classic example of a non-stoichiometric interstitial hydride.

Interstital compound are almost inert so (A) is wrong statement.

A interstitial compound has ions or atoms of a nonmetal occupy interstitial positions in a metal lattice. All of the three transition metal mentioned above have the tendency to form interstitial compounds.

Addition of non-metals like B and C to the interstitial sites of a transition metal results in the  more hardness so it will be less ductile and less malleable.
Hence options B,C & D are correct.