The solid NaCl is a bad conductor of electricity since

As per the given question, the correct answer is option C.

The solid NaCl is a bad conductor of electricity because ions in solid NaCl are bound by strong electrostatic force ,they are not free. In molten NaCl, ions are free to do motion and act as conductor of electricity.

 The correct option is C i.e In solid NaCl , there is no mobility of ions