Argon is the least reactive element.
Answer: Option 3
Step-by-step explanation:
The elements at the last group of periodic table has completely filled outermost shell, so they tend to be least reactive in nature with other elements. The elements in that last group is usually termed as Noble gases as the elements in the last group generally exhibit in gaseous state.
Argon is one of the Noble gas elements so it will be having least reactivity followed by potassium as it is an alkali metal, fluorine and sulfur have more reactivity than potassium and argon.