Schottky defect:
- In a Schottky defect, an equal number of cations and anions are missing from the crystal lattice, resulting in a vacancy defect.
- This defect does not affect the overall density of the crystal because the missing ions are compensated by the vacancies created.
- Since the number of missing ions is equal to the number of vacancies, the density remains the same.

Impurity defect:
- In an impurity defect, foreign atoms or ions are present in the crystal lattice in place of the host atoms.
- This defect can lead to a change in the density of the crystal depending on the size and mass of the impurity atoms.
- If the impurity atoms have a smaller size or lower mass than the host atoms, the overall density of the crystal may decrease.
- On the other hand, if the impurity atoms have a larger size or higher mass, the density may increase.

Frenkel defect:
- In a Frenkel defect, an ion is displaced from its regular lattice site to an interstitial site.
- This defect does not involve the creation of vacancies or the introduction of foreign atoms, so the density of the crystal remains unchanged.
- The displaced ion occupies an interstitial site within the crystal lattice, which does not affect the overall packing of atoms and therefore does not affect the density.

F-centre defect:
- In an F-centre defect, a cation is missing from its lattice site and an electron occupies an interstitial site.
- This defect can lead to a change in the density of the crystal, as the missing cation and the extra electron both contribute to the overall mass and volume of the crystal.

Therefore, among the given options, the Frenkel defect is the only defect in which the density of the crystal remains the same.

In Frenkel defect density of the crystal remains unchanged.