Final answer:
The least reactive element among those listed (Iodine, Chlorine, Argon, and Europium) is Argon (Ar), because it belongs to the noble gases which are known for their stability and very low reactivity.
Step-by-step explanation:
When looking at the reactivity of the elements given — 53-I (Iodine), 17-Cl (Chlorine), 18-Ar (Argon), and 63-Eu (Europium) — the least reactive element would be Argon (Ar). Argon is a noble gas, which means it has a full valence shell of electrons, making it very stable and unreactive.
Noble gases have very low reactivity because they do not tend to gain, lose, or share electrons, whereas Iodine, Chlorine, and Europium are more reactive as they are not in the noble gas family.