Step-by-step explanation:
The definition of buffer solution is: "a buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid.
• HNO₂/NO₂⁻:
HNO₂ is a weak acid and NO₂⁻ is its conjugate base. So they can form a bufffer solution.
• NH₃/NH₄⁺:
NH₃ is a weak base and NH₄⁺ is its conjugate acid. So they can form a bufffer solution.
• HCN/CN⁻:
HCN is a weak acid and CN⁻ is its conjugate base. So they can form a bufffer solution.
• HCl/OH⁻:
HCl is a strong acid and OH⁻ represents a strong base. So they can't form a bufffer solution.
Answer: HCl/OH⁻