Vacancy defect is a type of stoichiometric defect, where some of the lattice sites in the solid remain vacant. It results in the decrease in the density of the substance. Option (i) is correct.

Interstitial defect is also a type of stoichiometric defect where constituent particles such as atoms and ions occupy the interstitial spaces. This defect increases the density of the substance. Option (ii) is correct.

Impurity defect is a type of point defect, which is caused by the presence of an impurity in a crystal. For example, ionic impurities in ionic solids which have a different valence than the main constituent, cause formation of vacancies. It affects the density of the substance, thus option (iii) is incorrect.

Frenkel defect is a stoichiometric point defect seen in ionic solids. This defect causes both vacancy and interstitial defects. This occurs when a smaller ion, usually a cation is displaced from its original position to an interstitial site, causing a vacancy defect at the original position and an interstitial defect at the new position. Frenkel defect occurs in ionic solids where there is a huge size difference between cations and anions. It does not change the density of the solid. Hence (iv) is incorrect.