Answer
The least stable metal is Potassium
Explanation
Reactivity measures how readily a substance undergoes a chemical reaction. The reaction can involve the substance on its own or with other atoms or compounds, generally accompanied by a release of energy.
By observing the periodic table below we can see that potassium is an alkali metal, which will be more reactive than the other metal classifications presented. Potassium, in fact, is reactive, as it reacts very rapidly with water to form a colorless solution of potassium hydroxide (KOH) and hydrogen gas (H₂).
On the other hand, by observing the rest of the options we can see that Silver, a transition metal is the least reactive. I provide a graph at the end of the explanation where you can check a metal's relative reactivity.