Answer:
0.12 M hydrofluoric acid + 0.17 M potassium fluoride
Step-by-step explanation:
To make a buffer, you must to have an aqueous mixture of a weak acid and its conjugate base or vice versa.
Knowing that:
0.32 M calcium chloride + 0.27 M sodium chloride: is not a good buffer system because CaCl₂ and NaCl are both salts.
0.35 M ammonia + 0.36 M calcium hydroxide is not a good buffer system because ammonia is a weak base but calcium hydroxide is a strong base
0.19 M barium hydroxide + 0.28 M barium chloride is not a good buffer system because Ba(OH)₂ is a strong base.
0.12 M hydrofluoric acid + 0.17 M potassium fluoride is a good buffer system because HF is a weak acid and KF (F⁻ in aqueous medium), is its conjugate base
0.20 M hydrobromic acid + 0.22 M sodium bromide is not a good buffer system because HBr is a strong acid.