Answer:
Helium
Step-by-step explanation:
Hi, Helium is the only one in that list that belongs to the noble gases (group 18 of the periodic table).
This gases have the characteristic of being stable in a monoatomic molecule
Why? Becuase they have their last atomic level with a completed octet of electrons and cause of that, they don't have the need to gain, lose or share electrons with other atoms.