Identify the polyatomic ions from the following.

The correct options are
A

$N{H}^{4+}$

D

Hydroxide ion

An ion is an atom with positive or negative charge over it. A polyatomic ion is the one which has more than one atom within the ionic group. The formula of hydroxide ion is $O{H}^{-}$. $O^{-}$ and $H^{-}$ make it polyatomic. Similarly, $N{H}^{4+}$ is also a polyatomic ion.
Note: Don't get confused between the charge on the ion and the atomicity. Here $A{l}^{3+}$ and $O^{2-}$ have +3 and -2 charges respectively, but each one is a single atom.