Which among the following non-metal is "monoatomic"?

The correct answer is Option (4) → Helium

A monoatomic element exists as single atoms. Helium is a noble gas: its only shell holds 2 electrons and is full, so a helium atom does not need to share electrons and stays alone.

Oxygen, chlorine and nitrogen are diatomic: two atoms share electrons to complete their outer shells, giving the molecules O₂, Cl₂ and N₂.