Which of the following aqueous solutions are good buffer systems?

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0.11 M sodium hydroxide + 0.30 M sodium bromide

0.33 M potassium chloride + 0.20 M calcium chloride

0.28 M nitric acid + 0.23 M sodium nitrate

0.31 M ammonia + 0.31 M calcium hydroxide

0.21 M hydrofluoric acid + 0.20 M potassium fluoride

Buffer is composed of weak acid and its conjugate base or weak base and it is conjugate acid. so The correct answer is:

0.21 M hydrofluoric acid + 0.20 M potassium Fluoride

due to the first choice contain NaOH which is strong base not weak

second choice both salts no acid or base

Third choice contain HNO₃ which is considered as strong acid

Fourth choice contain ammonia but without its conjugate acid like (NH₄Cl for example) instead it has different salt

Fifth Choice contain HF which is weak acid and KF which is its conjugate base so this is the correct one