Hello!
The solution that is a buffer is a solution that is 0.100 M in HNO₂ and 0.100 M in NaNO₂
The condition for a buffer solution is that it needs to be formed of a weak acid and its conjugate base, or a weak base and its conjugate acid. From the list, the only set of compounds that is an Acid and its conjugate base is HNO₂ (Weak Acid) and NaNO₂ (Conjugate Base). The pH of this solution would be the same as the pKa of HNO₂ (3,15) since both concentrations are equal.
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