Answer
The conjugate base of $HS{O_4}^-$ is $S{O_4}^{2-}$.
Work Step by Step
To find the conjugate base of an acid, just remove one proton $(H^+)$ from the compound.
1. Remove one hydrogen:
$HS{O_4}^- -- \gt S{O_4}^-$
2. Subtract one from the charge
$-1 --\gt -2$
${SO_4}^{-} -- \gt S{O_4}^{2-}$