The question requires us to talk about monoatomic ions.
First, we need to keep in mind that an ion consists in an atom or group of atoms that carries a positive or negative charge as a result of losing or receiving electrons.
A monoatomic ion is an ion that consists in just one atom. On the other hand, polyatomic ions are composed of more than one atom. We can identify a monoatomic ion when there is a charge (positive or negative) over just one atom.
For example: Na+ is a monoatomic ion, just like Ca2+, Cl- and O2-. On the other side, NO3-, SO42- and NH4+ are examples of polyatomic ions.