What defines a buffer solution?

A buffer solution is defined as a solution that resists changes in pH upon the addition of small amounts of acid or base. Buffers typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid. This combination allows the buffer to neutralize added acids and bases effectively, maintaining a relatively stable pH level. This characteristic is crucial in various biological and chemical processes where maintaining a specific pH range is essential for function and stability.

The other definitions provided do not accurately describe a buffer solution. For example, the ability to change temperature easily does not relate to buffering capacity, nor do strong acid solutions inherently resist pH changes. Similarly, a solution that evaporates quickly also lacks relation to the concept of pH stability. Therefore, the essence of a buffer lies in its ability to maintain pH despite external influences, which is precisely captured in the correct choice.