Answer
The conjugate base of $HS{O_3}^-$ is $S{O_3}^{2-}$.
Work Step by Step
To find the conjugate base of an acid, just remove one proton $(H^+)$ from the compound.
1. Remove one hydrogen:
$HS{O_3}^- -- \gt SO{_3}^-$
2. Subtract one from the charge
$-1 --\gt -2$
$S{O_3}^- -- \gt S{O_3}^{2-}$
