A buffer must contain one of two conjugate pairs in about equal ammounts
- A weak acid and its conjugate base, or
- A weak base and its conjugate acid.
So, right off the bat, if you spot a strong acid or a strong base in one of the pairs, it will automatically mean that you're not dealing with a buffer.
Upon inspection of the options given to you, you can see that you are indeed dealing with a strong acid in one of the pairs; more specifically, the last pair contains hydroiodic acid,
Now for the other three.
This pair contains hydrogen cyanide,
#underbrace(HCN_((aq)))_(color(blue)("weak acid")) + H_2O_((a)) rightleftharpoons H_3O_((aq))^(+) + underbrace(CN_((aq))^(-))_(color(green)("conjugate base"))#
Since it contains both a weak acid, and its conjugate base, presumably in equal amounts, this solution will act as a buffer.
This one is a little more difficult to spot. You're dealing with two salts, monosodium phosphate,
However, in aqueous solution, the two salts will dissociate completely to give
#NaH_2PO_(4(aq)) -> Na_((aq))^(+) + H_2PO_(4(aq))^(-)#
#Na_2HPO_(4(aq)) -> 2Na_((aq))^(+) + HPO_(4(aq))^(2-)#
Notice that you've discovered dihydrogen phosphate,
As a result, this solution will also act as a buffer.
This time you're dealing with ammonia,
#underbrace(NH_(3(aq)))_(color(blue)("weak base")) + H_2O_((l)) rightleftharpoons OH_ ((aq))^(-) + underbrace(NH_(4(aq))^(+))_(color(green)("conjugate acid"))#
This solution will also act as a buffer.