The correct option is D Packing fraction remains unaffected in vacancy defect
(a) Schottky defects are formed due to missing of equal number of cations and anions from a crystal lattice. Frenkel defects are known as dislocation defects. Hence this statement is wrong.
(b) When Al3+ is inserted in NaCl, two cation vacancies are created. After the insertion of Al3+ in place of one Na+, the crystal lattice has +2 charge. To maintain electroneutrality, two Na+ ions must be removed and hence only two vacancies are created.
c) As has one more valence electron than Si, so addition of As to Si results in formation of n-type semiconductors.
d) Packing fraction is the fraction of volume occupied in a crystal structure. Hence, due to vacancy defect, packing fraction decreases.