Answer
(a) Conjugate acid-base pair.
(b) Not a conjugate acid-base pair.
(c) Conjugate acid-base pair.
(d) Not a conjugate acid-base pair.
Work Step by Step
A conjugate acid-base pair has exactly one $H^+$ as the difference in their formulas.
(a) $NH_3$ and $NH_4^+$: Exactly one proton as the difference.
(b) $HCl$ and $HBr$: Chlorine and Bromine as the difference. Not a conjugate acid-base pair.
(c) $C_2H_3O_2^-$ and $HC_2H_3O_2$: Exactly one proton as the difference.
(d) $HCO_3^-$ and $NO_3^-$: Carbon and Nitrogen as the difference. Not a conjugate acid-base pair.
