Answer
${O_2}^{2-}$ is a strong base.
Conjugate acid: $OH^-$ which has negligible acidity.
Work Step by Step
${O_2}^{2-}(aq) + H_2O(l) --> 2OH^-(aq)$
As we can observe, $OH^-$ is the conjugate acid of ${O_2}^{2-}$.
Since ${O_2}^{2-}$ is a strong base, his conjugate acid will have negligible acidity.
