In an aqueous solution, a significant role played by pH buffers is to:

• A. Increase the rate of all acid–base reactions by acting as a catalyst
• B. Resist changes in pH when small amounts of acid or base are added
• C. Permanently fix the pH at exactly $7$ regardless of what is added
• D. Convert strong acids into strong bases (and vice versa) so the solution becomes neutral

Answer: (B)

Verified step by step guidance

1. Understand what a pH buffer is: A pH buffer is a solution that contains a weak acid and its conjugate base (or a weak base and its conjugate acid) which helps maintain the pH of the solution.
2. Recognize the main function of a buffer: Buffers resist changes in pH when small amounts of acid or base are added by neutralizing the added H\textsuperscript{+} or OH\textsuperscript{−} ions.
3. Eliminate incorrect options: Buffers do not increase the rate of all acid-base reactions (they are not catalysts), do not permanently fix pH at exactly 7, and do not convert strong acids into strong bases.
4. Focus on the correct role: The key role of buffers is to maintain pH stability, which means they resist changes in pH upon addition of small amounts of acid or base.
5. Summarize: Therefore, the correct understanding is that buffers help resist changes in pH when small amounts of acid or base are added to the solution.