Frenkel defect occurs in those ionic compounds in which the sizes of cation and anion are largely different, so the statement about Frenkel defect is incorrect. Non-stoichiometric ferrous oxide is $Fe_{0.93-0.96}O_{1.00}$ and it is due to metal deficiency defect, so the formula $FeO_{0.98}$ in the statement is also incorrect. Note: the source key accepts both the $FeO_{0.98}$ statement and the Frenkel-defect statement as incorrect statements.