The reactivity of metals is determined by their position in the reactivity series (or activity series). Metals at the top are most reactive, and those at the bottom are least reactive.
Let's analyze the given options based on the reactivity series:
K (Potassium): It is an alkali metal, located at the very top of the reactivity series. It is extremely reactive and reacts violently with water.
Al (Aluminium): It is a fairly reactive metal, but it typically forms a thin, tough, protective layer of aluminum oxide on its surface that prevents further reaction. It is less reactive than Potassium.
Ag (Argentum/Silver) and Au (Aurum/Gold): These are noble metals, found near the bottom of the reactivity series. They are very unreactive and do not easily corrode or react with acids.
Comparing the four elements, Potassium (K) is by far the most reactive.