Answer
(a) $H_2CO_3$
(b) $HC{O_3}^-$
(c) $H_2O$
(d) $H_3PO_4$
Work Step by Step
The conjugate acid is the result of a base that receives one proton, therefore, you just need to add one hydrogen to the chemical formula, and sum 1 to the charge.
For example:
1. Add the hydrogen:
$H{CO_3}^- -- \gt H_2{CO_3}^-$
2. Sum 1 to the charge:
$H_2{CO_3}^- -- \gt H_2CO_3$