Final answer:
Argon is the most stable element out of the ones listed due to its complete electron shells and noble gas properties, making it chemically inert.
Step-by-step explanation:
Among the elements listed - argon, manganese, phosphorus, and rubidium - argon is the most stable element. This is because argon is a noble gas and its atoms have complete electron shells, which makes them chemically inert and stable under normal conditions. In contrast, manganese and phosphorus are reactive elements, and rubidium, which is an alkali metal, is also highly reactive. Furthermore, it's important to note that in terms of nuclear binding energy, which determines the stability of an element's nucleus, iron is considered the most stable element overall. However, argon is the most stable among the options provided in this particular context.