Which statement best describes a buffer solution?

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A buffer solution resists changes in pH by containing a weak acid and its conjugate base in solution. When a small amount of strong acid is added, the conjugate base neutralizes the extra H+, forming the weak acid and water, which keeps the pH from dropping quickly. Conversely, when a small amount of base is added, the weak acid donates a proton to neutralize OH−, forming its conjugate base and water, which prevents a rapid pH rise. This is why the statement that a buffer contains a weak acid and its conjugate base best describes a buffer. A strong acid solution would drive pH downward without buffering; a metal–nonmetal combination isn’t related to buffering behavior; and having zero pH is not a general property of buffers—pH depends on the specific acid–base pair and can vary. For a practical sense, buffering is most effective near the pKa of the weak acid, and the relationship is captured by pH ≈ pKa + log([A−]/[HA]).