The correct answer is D – Mineral Water.
Besides mineral water, all the above serve as rather good insulators: Air can conduct electricity to a certain level (lightning does cross the skies), yet if it were a good conductor, we would get electrified all day long, and no battery would be able to preserve its electric charge.
Plastic and Glass are known common insulators.
Pure water is water containing no dissolved substance. The ions in electrolytes conduct electricity, and not the water molecules themselves. Pure water contains no dissolved substance, hence no ions; therefore, it doesn't have the means to conduct electricity. Mineral water on the other hand, has an abundance of ions, and serves as a great conductor.