Correct option is (b) Free ions
Molten sodium chloride conducts electricity due to the presence of free ions.
In order to conduct electricity a substance must have charge particles, such as electrons and ions, that are free to move freely through it. In the solid state, ionic compounds such as sodium chloride have their ions fixed in position and therefore these ions cannot move so solid ionic compounds cannot conduct electricity. However in the molten state, ions in ionic compounds are free to flow and therefore molten sodium chloride can conduct electricity.
