Wich is the incorrect statement?

$Fe{O}_{0.98}$ has non-stoichiometric metal excess defect. It occurs due to missing of a negative ion from its lattice site, thus leaving a hole which is occupied by an electron. Non-stoichiometric ferrous oxide is $Fe{O}_{0.93-0.96}$ and it is due to metal deficiency defect. Thus, statement (a) is incorrect.
(b) In an ionic crystal of $A^{+}{B}^{-}$ types, if equal number of cations and anions are missing from their lattice sites, the defect is called Schottky defect. Due to such defect, density of solid decreases. Thus, statements (b) is correct.
(c) $NaCl$-insulator, Silicion (Si) - semiconductor, Silver (Ag) - conductor, Quartz - piezoelectric crystal.
Thus, statement (c) is correct.
In an ionic crystal when an ion is missing from its lattice site and occupies interstitial site, the defect is called Frenkel' defect. This type of defect is seen in those crystals where the difference in the size of cations and anions is very large and their coordination number is low Thus, statements (d) is incorrect.