What is a solution that resists changes in pH?

A buffer solution is designed specifically to resist changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base (or a weak base and its conjugate acid), which work together to neutralize the addition of additional hydrogen ions (H⁺) or hydroxide ions (OH⁻). This balancing act maintains the pH within a narrow range, making buffers essential in biological systems, chemical reactions, and various laboratory procedures where maintaining a stable pH is crucial.

In comparison, acids and bases alone can change the pH significantly when introduced to water or other solutions. An assay refers to a procedure in laboratory settings to measure or quantify a particular substance but does not inherently have properties that stabilize pH. Thus, the buffer is the only solution among the options that specifically serves the purpose of resisting pH fluctuations.