Which set of compounds would form a buffer in aqueous solution? drag each item to the appropriate bin. hints?

This question requires a list of set of compounds (answer choices) from which choose the set that would form a buffer is aqueous solution.

I found this list:

1) HCN and NaCN

2) HCN and HCl

3) HCl and NaOH

4) NaCN and NaOH

Now you must apply the rules to determine which pairs may form a buffer solution.

A buffer solution is a mixture of a weak acid with its conjugate-base or a mixture of a weak base with its conjugate base.

You can disscard quickly 2, 3, and 4 because HCl is a strong acid, and NaOH is a strong base.

So, the only option is the first one:HCN and NaCN.

Indeed HCN is a weak acid and CN - is its conjugate base, so they will form a buffer.

Other set of compounds that would form a buffer, and tha is frequent in this kind of questions, is:

HF (a weak acid) and NaF (F - is the conjugate-base of HF).