According to the bronsted-lowry theory, Conjugate acid-base pair is the species which differs by only one proton i.e. when you remove a proton from a parent acid it will become a conjugate base of that acid and that acid will be called a conjugate acid of that base.

(i) NH₃ - (b) NH⁺⁴

The conjugate acid of a base is formed by the protonation (reception of an H⁺) of the acid. When an ammonium molecule accepts a proton (H⁺)), the ammonium cation (NH₄⁺) is formed, as the nitrogen atom forms a coordinate bond with the hydrogen.

Explanation: The conjugate acid of ammonia is ammonium ion.

(ii) HCO₃^-- (e) H₂CO₃

Explanation: The bicarbonate ion(HCO₃^-) is a negatively charged base HCO3-, when we add a proton (H⁺)i.e. positively charged species on reacting with water we get the neutral conjugate acid, H₂CO_3.

(iii) H₂O - (c) H₃O⁺

Explanation: when water reacts with acid it acts as a base. When the proton (H⁺) will associate with water molecule it will form a hydronium ion ().

(iv) HSO₄^- - (d) H₂SO₄

Explanation: The conjugate acid of hydrogen sulphate ion is H₂SO_4.when hydrogen sulphate ion gains a proton i.e. H⁺ it gets converted into sulphuric acid (H₂SO₄).