Which of the metals listed in the options is the least reactive? 

The correct answer is Gold.

Key Points

• The least reactive metal is gold. The chemical element gold has the symbol Au and the atomic number 79.
• It is a bright, somewhat reddish yellow, dense, soft, malleable, and ductile metal in its purest form.
• Gold is a transition metal and a group 11 element chemically.
• A Reactivity series is an arrangement of metals in decreasing order of reactivity.
• In a displacement reaction, this series is used to assess if a given metal can displace another metal. 
• Based on their reactions with water and dilute acids, the metals potassium, sodium, lithium, calcium, magnesium, zinc, iron, and copper can be arranged in decreasing order of reactivity. Iron is more reactive than copper.
• Because iron has four unpaired shells in its three subshells, while copper only has one, according to the atomic structure. As a result, iron reacts more quickly than copper.