(A) The absence of an equal amount of cations and anions from a crystal lattice results in the formation of Schottky defects. Dislocation defects are often referred as as Frenkel defects. Therefore, this assertion is false.

(b) Two cation vacancies are produced when ${\mathrm{Al}}^{3+}$ is added to $\mathrm{NaCl}$. The crystal lattice now bears ${\mathrm{Na}}^{+2}$ charge after one ${\mathrm{Na}}^{+}$ was replaced with an ${\mathrm{Al}}^{3+}$. Only two vacancies are produced because only two ${\mathrm{Na}}^{+}$ ions need to be eliminated in order to preserve electroneutrality.

 (c ) Because As has one extra valence electron than Si, it can be added to Si to produce n-type semiconductors.

d) The packing fraction measures the percentage of a crystal structure's volume that is occupied. Therefore, packing fraction declines as a result of the vacancy defect.

hence option (b) is incorrect