The correct (English) version of this sentence suggests that we don't know what copper is; that's why we are asking.
However, from the Greek sentence I understand Which one is the copper?. I would ask this question in the presence of a few metal objects to find out which one is made of copper.
I would have expected to see τι instead of ποιος here. Why is ποιος used?
The original Latin word for copper was æs, but that word was extended to its alloy with tin, bronze, and as this was far more extensively used than pure Copper, the word's primary sense shifted to the alloy and a new word evolved for Copper, from the Latin form of the name of the island of Cyprus (in Greek Κυπρος [Kypros]), where copper was mined: Cyprium (æs) (Greek: χαλκος κυπριος [chalkos kuprios]). http://www.vanderkrogt.net/elements/element.php?sym=Cu