A buffer solution is a solution that resists changes in pH when small amounts of an acid or a base are added. This occurs because a buffer solution typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid.
In the options given:
- A mixture of nitric acid and sodium nitrate: This does not form a buffer solution because nitric acid is a strong acid, and strong acids do not form buffer solutions with their conjugate bases.
- A mixture of sodium hydroxide and nitric acid: This does not form a buffer solution as it is a neutralization reaction between a strong base and a strong acid.
- A mixture of acetic acid and sodium hydroxide: This can form a solution where acetic acid is partially neutralized to form sodium acetate, but it won't function as a buffer until all reactants are in the correct stoichiometric proportions.
- A mixture of acetic acid and sodium acetate: This is a classic example of a buffer solution. Acetic acid (a weak acid) and sodium acetate (its conjugate base) together form a buffer solution that can resist changes in pH.
Therefore, the correct answer is: A mixture of acetic acid and sodium acetate.