Potassium is a highly reactive metal that belongs to the alkali metals group. It is soft, silvery-white in color, and has a low melting point. Potassium is highly flammable and reacts vigorously with water, releasing hydrogen gas and forming potassium hydroxide.

In contrast, aluminum, silver, and iron are relatively stable metals. Aluminum is a lightweight metal that is resistant to corrosion and has a high strength-to-weight ratio. Silver is a precious metal that is highly conductive and has a lustrous appearance. Iron is a strong and durable metal that is used in a wide variety of applications, such as construction, transportation, and machinery.

Therefore, potassium is the least stable metal among the given options.