Final answer:
An element with an atomic number of 118 and 8 valence electrons would have reactivity most similar to Argon (Ar), a noble gas, due to a full valence shell.
Step-by-step explanation:
The element with an atomic number of 118 and 8 valence electrons is most likely to belong to the noble gases, which are known for being inert or unreactive due to a full valence shell. Looking at the choices given, the element's reactivity will be most like Argon (Ar), which is also a noble gas with a complete valence shell. The other elements listed (Rubidium, Iodine, and Lead) do not have this property; Rubidium is an alkali metal, Iodine is a halogen, and Lead is a post-transition metal, all of which have different reactivity patterns compared to noble gases.