Which of the following elements is found in nature only as oxide minerals?

I think Al is the best answer although it is found in many silicate minerals. Pb and Hg form sulfides also. Au and Ag are typically native elements.

All of the metals listed except mercury, which is liquid at room temperature, are protected from further oxidation by an initial thin layer of oxide. For this reason I believe mercury could not long exist in nature without forming an oxide. Each of the others could be smelted by volcanic action and continue to exist in relatively pure lumps.