NaCl crystals are ccp structures, and a property of ccp structures is that they contain 4 atoms per unit cell

The number of tetrahedral voids is twice the number of atoms per unit cell, and number of octahedral voids is equal to the number of atoms per unit cell.

Hence, NaCl contains 4 × 2 = 8 tetrahedral voids and 4 octahedral voids. The correct answer is (ii) and (iii).