Which of the following metals is the best electrical conductor, but isnt used in wire because of its extremly high cost?

In the absence of choices above, the best conductor is pure silver with 105% conductivity rating. This could be a replacement for copper wires but pure silver is more expensive than copper; it also tarnishes easily. 

Gold, which is believed to be a good conductor, is far less conductive than copper and even much lesser conductive than pure silver. 

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