Which of the following represents a Brønsted-Lowry conjugate acid-base pair?

Question

SO3^2- and SO3

CO3^-2 and CO

H3O and H2

NH4+ and NH3

Answer 1

NH4+ and NH3 Pair

NH4+ is Proton Donor  ( Acid )

NH3 is Proton Acceptor  ( Conjugate Base )

Other pairs :  

First and Second Pairs do not have Proton Donor

H2 in Third Pair is not a Proton Acceptor

Answer 2

ACID-BASE pairs are just that - acids that give a base when H+ is removed. There is only one here that satisfies that. All the others are redox pairs.

The acid base pair for H3O+ is H3O+ : H2O