Which of the following does not conduct electricity?

The correct option is C

Solid $\mathrm{NaCl}$

Part 1: Explanation for correct answer:-

Option C) Solid $\mathrm{NaCl}$ :-

1. Solid NaCl does not have free electrons or ions, thus it cannot conduct electricity.
2. But it does conduct electricity in molten or in solution form
3. Hence it is the correct option.

Final answer:- Option (C) Solid $\mathrm{NaCl}$ cannot conduct electricity.