What is the conjugate base of ${\mathrm{OH}}^{-}$ ?

Conjugate base is the term for the portion of an acid that remains after it produces ${\mathrm{H}}^{+}$.
Therefore, if we take ${\mathrm{H}}^{+}$ out of ${\mathrm{OH}}^{-}$, we obtain ${\mathrm{O}}^{2-}$.

${\mathrm{OH}}^{-}\to {\mathrm{O}}^{2-}+{\mathrm{H}}^{+}$

${\mathrm{O}}^{2-}$ is hence the conjugate base of ${\mathrm{OH}}^{-}$.

The choice is D as a result.