What is the conjugate base of OH$$^-$$?

A conjugate base is obtained by removing one proton ($$H^+$$) from the given species. The general rule is

acid $$\rightleftharpoons$$ conjugate base $$+$$ $$H^+$$

For the hydroxide ion $$OH^-$$, remove one $$H^+$$:

$$OH^- \; \rightarrow \; O^{2-} + H^+$$

The species produced after losing the proton is $$O^{2-}$$, called the oxide ion. Hence, $$O^{2-}$$ is the conjugate base of $$OH^-$$.

Option D which is: $$O^{2-}$$