* Polyatomic ions are groups of atoms that act as a single charged unit.

Let's examine each:

* Hydroxide: OH⁻ - This is a polyatomic ion (one oxygen and one hydrogen atom) with a -1 charge.

* Chloride: Cl⁻ - This is a monatomic ion (single chlorine atom) with a -1 charge.

* Ammonium: NH₄⁺ - This is a polyatomic ion (one nitrogen and four hydrogen atoms) with a +1 charge.

* Carbonate: CO₃²⁻ - This is a polyatomic ion (one carbon and three oxygen atoms) with a -2 charge.

Therefore, the polyatomic ions in your list are hydroxide, ammonium, and carbonate.