Answer
The conjugate acid of $C{O_3}^{2-}$ is $HCO{_3}^-$
Work Step by Step
To find the conjugate acid of a base, you just need to add a proton $(H^+)$ to the compound.
1. Add one hydrogen.
$C{O_3}^{2-} -- \gt HC{O_3}^{2-}$
2. Add "+1" to the charge of the compound:
$HC{O_3}^{2-} -- \gt HC{O_3}^{-}$