Interstitial defect occurs when some constituent atoms or molecules occupy an interstitial site in the lattice. This defect increases the density of the substance it occurs in. Vacancy defect occurs when sites in the lattice are vacant, and this reduces the density of the solid. Frenkel Defect is when the smaller ion (usually cation) is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location. This does not change the density. Schottky defect is like a vacancy defect in ionic solids and this also decreases the density of the solid.

Thus, vacancy defect and Schottky defect decrease the density of solid. The correct options are (ii) and (iv).