Which of the following is the least reactive metal? 

Gold 

The correct answer is option 3. Gold.

Among the options provided, gold is the least reactive metal. Gold belongs to the group of noble metals, which are known for their low reactivity. It is relatively unreactive under normal conditions and does not readily react with oxygen, water, acids, or most other chemicals. This property makes gold highly valuable for various applications, including jewelry and currency, as it maintains its luster and does not corrode or tarnish easily. So, the correct answer is Gold.