One small circle sits inside a big circle, and a third circle cuts only the big one without touching the small one. Which group of three does the diagram represent?

Iron is a metal (small circle inside Metals). Mercury is a metal that is a liquid, so Liquids overlaps Metals, but iron is not a liquid, so Liquids does not touch Iron. Copper would sit inside Metals, iron sits inside magnetic things, and stars are not planets at all.