Concept: Reactivity of metals decreases down the electrochemical series for noble metals; the least reactive metal is the most stable and unreactive.Explanation:Among the options, lead (Pb), tin (Sn), and mercury (Hg) are moderately reactive metals.Gold (Au) is a noble metal and does not readily react with oxygen, water, or dilute acids.Therefore, gold is the least reactive metal.Answer: D. Au