There are two criteria for your answer. What is a nonmetal? What is not an electrical conductor? What is left?
As a follow-up to PsyDAG's answer, a non-metal is an element which, quite simply, is not a metal. These non-metals include Oxygen, Nitrogen, and Chlorine to name a few. Copper, however, is a metal, and therefore copper wire cannot be correct as the question asks for a non-metal conductor. Plastic covering of a wire therefore can also not be correct - as wires are made from metal. The two that are left are salt water and cotton clothing. Now you must ask yourself, which of the two is a conductor of electricity. Cotton certainly is not (you don't get an electric shock when you touch a cotton shirt during winter, or if your shirt comes in contact with electricity, for example) - and that leaves salt water, which is your answer - a non-metal conductor.
