Okay we can very quickly rule out three of these answers. Gold, iron and sulfur are all mined from the ground. The reason gold is so expensive is because it is rare and hence precious, if it was renewable there would be lots available and it wouldn't be expensive.
Iron is mined from various minerals in rock, and undergoes various industrial processes to be converted into useful metal. Again it is not renewable because iron is only formed in the core of stars later in their life cycles, and in supernovae. These are stellar events, and we cannot sustainably reproduce these on Earth.
Sulfur is often obtained from volcanoes, where it erupts from deeper inside the earth. We cannot produce it.
This leaves oxygen. If it wasn't renewable, we would have used it all up by now and nobody would be able to breathe and survive. It is produced by trees and plants, where photosynthesis converts atmospheric carbon dioxide, and water, into oxygen and glucose, for energy to the plant. This is one of the many reasons deforestation is so destructive. Trees are carbon sinks, so remove the greenhouse gas CO2 from the air.
I hope this helps you :)