Schottky defect is a type of point defect that occurs in ionic crystals. It is characterized by the presence of vacancies in both the cation and anion lattice sites of the crystal structure. This defect is named after the German physicist Walter H. Schottky, who first described it.

Here is an explanation of the different defects mentioned in the question and why Schottky defect is the correct answer:

a) Vacancy defect:
- Vacancy defect occurs when an atom or ion is missing from its expected lattice position in the crystal structure.
- This defect leads to the formation of a vacancy, which is an empty lattice site.
- In the case of Schottky defect, both cation and anion vacancies are present, resulting in a charge-neutral defect.

b) Compositional defect:
- Compositional defect occurs when there is a variation in the stoichiometry or composition of the crystal.
- It can be caused by the substitution of one type of atom or ion with another type.
- An example of compositional defect is the substitution of a divalent cation with a trivalent cation, leading to a charge imbalance in the crystal structure.
- Schottky defect is not a compositional defect as it does not involve substitution of atoms or ions.

c) Interstitial defect:
- Interstitial defect occurs when an extra atom or ion occupies an interstitial site, which is the space between the regular lattice sites.
- This defect leads to an increase in the density of the crystal structure.
- Schottky defect is not an interstitial defect as it involves vacancies rather than extra atoms or ions.

d) Surface defect:
- Surface defect occurs at the surface of a crystal and involves deviations from the regular crystal structure.
- It can be caused by surface roughness, impurities, or dislocations.
- Schottky defect is not a surface defect as it occurs within the crystal lattice rather than at the surface.

In summary, Schottky defect is a vacancy defect characterized by the presence of vacancies in both cation and anion lattice sites. It is not a compositional defect, interstitial defect, or surface defect.