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