Final answer:
The combination that is NOT a buffer solution is C. HBr and KBr because HBr is a strong acid and KBr is its salt, which cannot create a buffer.
Step-by-step explanation:
The combination that is NOT a buffer solution is C. HBr and KBr. A buffer solution is composed of a weak acid and its conjugate base, or a weak base and its conjugate acid. The combinations HCN and NaCN and CH₃COOH and NaCH₃COO both involve a weak acid and its conjugate base, thus they can create buffer solutions. NH₃ and (NH₄)₂SO₄ involve a weak base and the salt of its conjugate acid, which also qualifies as a buffer. In contrast, HBr is a strong acid, and KBr is a salt that has no buffering capability with its conjugate acid, making it unable to form a buffer solution.