Which of the following pairs contains a conjugate acid-base pair?

Hydrochloric acid and sodium hydroxide

Sodium cyanide and hydrocyanic acid

sodium bromide and bromous acid

Remember that a conjugate acid-base pair will differ only by one proton.

None of the options you listed are conjugate acid-base pairs as none of them differ only by one proton (or H⁺)

An example of a conjugate acid-base pair would be NH₃ and NH₄⁺NH₃ + H₂O - - > NH₄⁺ + OH⁻NH3 is the base, and NH₄⁺ is the conjugate acid