Which of the following metals is the least reactive?

Correct answer: Au (gold).

Explanation: Gold is a noble metal—it sits very low on the reactivity (activity) series and resists corrosion and oxidation. It does not react easily with acids or oxygen, which is why it is found native and used in jewelry.

• Hg (mercury): Relatively unreactive compared with many metals, but still more reactive than gold.
• Pb (lead) and Sn (tin): Both are above Hg and Au in the activity series; they react more readily (e.g., with acids) and corrode more easily.

Key idea: The lower a metal is on the activity series, the less reactive it is; Au is among the least reactive of all metals.