How to identify if it is a buffer solution
Well, it all boils down to the preparation of buffer solutions in the first place. One of the methods of preparation includes the mixing of a salt solution of an weak acid and a strong base with a solution of a strong acid, with the solution of the strong acid as a limiting reagent, which leads us to conclude positively for the third choice.

Alternatively, you can think which of the mixtures will resist the change in pH on mixing a strong acid or a strong base; for instance in the third choice, nitric acid being a strong acid will help to resist the change in pH when a strong base is added, and $\ce{CH3COONa}$ being a weak acid + strong base type of salt resists the change in pH that could've been caused due to the addition of a strong acid.