Answer: Option A
:
A and D
$\bullet$ A monoatomic ion is an ion consisting exactly one atom. While a polyatomic ion is an ion consisting more than one atom.
$$\begin{array}{ccc}\text{Ion}& \text{Formula}& \text{No. of Atoms}\\ \text{Ammonium}& N{H}_4^{+}& \text{1 N, 4H}\\ \text{Hydride}& H^{-}& \text{1 H}\\ \text{Oxide}& O^{2-}& \text{2 O}\\ \text{Hydroxide}& O{H}^{-}& \text{1 H, 1 O}\end{array}$$
$\bullet$ Thus, $O{H}^{-}$ and $N{H}_4^{+}$ are polyatomic ions and $O^{2-}$ and  $H^{-}$ are monoatomic ions.