Solution

The schottky defect is a vacancy defect (due to missing ions) and also a stoichiometric defect, as the ratio of the number of cations and anions remains the same. (Schottky defects have an effect on the physical properties of solids). It occurs only when there is a small difference in size between cations and anions. This is produced as the result of the thermal incorporation of unoccupied lattice sites from the exterior of the crystal. The lattice undergoes thermal vibration and thermal expansion when the temperature is raised above $$0$$ K. When it happens the pair of vacancies are incorporated in the crystal.

A Frenkel defect, Frenkel pair or Frenkel disorder is a type of point defecting a crystal lattice. The defect forms when an atom or smaller ion(usually cation) leaves its place in the lattice, creating a vacancy and becomes an interstitial by lodging in a nearby location. In a Frenkel defect, an additional ion exists in the interstitial space within a crystal lattice. In a Schottky Defect, there is an empty space in a crystal lattice.

An F-center, Farbe center or color center (from the original German Farbzentrum; Farbe means color and Zentrum center) is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more unpaired electron (Trapping of an electron in the lattice leads to the formation of F-center).

Hence, options B and C are correct.