Concept: Electrical conductivity in ionic compounds depends on the mobility of ions. In the solid state, ions are held in fixed lattice positions, while in the molten state or in solution, they become free to move.

Why (A) is correct: In the solid state, sodium chloride (NaCl) exists as a crystal lattice where Na⁺ and Cl^- ions are held in fixed positions by strong electrostatic forces. They are not free to move and thus cannot carry an electric current. When NaCl is melted (molten state) or dissolved in water, these ions become mobile and can move towards the electrodes, thereby conducting electricity.

Option Analysis:

• A) The ions of NaCl become mobile only in molten state and are not free to move in solid state: This statement accurately explains why solid NaCl does not conduct electricity. The immobility of ions in the solid lattice prevents charge transfer.
• B) The crystalline structure does not have ions: This is incorrect. Sodium chloride is an ionic compound, and its crystalline structure is explicitly made up of Na⁺ and Cl^- ions.
• C) When a bond is formed between ions they lose their charges: This is incorrect. Ionic bonds are formed by the electrostatic attraction between oppositely charged ions. The ions retain their charges; they do not lose them upon bond formation.
• D) None of these: Since option A is correct, this option is incorrect.

Correct Answer: (A)