Ionic solids are a type of crystalline solids, where the constituent particles are ions. When voids are generated during formation of crystal structure, anions, which are the bigger ions (because of the repulsion of the electrons in the shells) form the close packed structure and smaller ions, the cations occupy the voids. Option (i) is correct.

Continuing the previous statement, not all tetrahedral or octahedral voids are occupied, if the smaller ions are small enough, they occupy the tetrahedral voids, otherwise they fit in octahedral voids. Option (ii) is correct.

The fraction of occupied tetrahedral voids to octahedral voids depends on the chemical formula of the substance and it is not necessary that all voids are occupied. Option (iii) is correct.

The fraction of octahedral or tetrahedral voids does not depend on the radii of the ions occupying the voids but the chemical formula of the substance. Option (iv) is incorrect.