Even though pure water is not a good conductor of electricity,some tap water is. What could explain this?

Gillian, "D" would be the best answer since not all tap water is going to contain iron. There are other impurities in water that make it conductive. You might be surprised at just what is in the water that you drink, if it is not well filtered water. While "dirty water" may sound nasty, it is a better description than the other answers in your list since technically, regardless of how clear it looks, it is "dirty".