A buffer solution:

### What is a Buffer Solution? A **buffer solution** is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. Its primary purpose is to resist changes in hydrogen ion concentration ($pH$) when small amounts of acid or base are added to it. ### How it Works Consider an acidic buffer made of acetic acid ($CH_3COOH$) and its conjugate base, acetate ($CH_3COO^-$): 1. **Adding Acid ($H^+$):** The added $H^+$ ions react with the conjugate base to form more weak acid: $$CH_3COO^- + H^+ \rightarrow CH_3COOH$$ This prevents the $H^+$ concentration from increasing significantly. 2. **Adding Base ($OH^-$):** The added $OH^-$ ions react with the weak acid to form the conjugate base and water: $$CH_3COOH + OH^- \rightarrow CH_3COO^- + H_2O$$ This prevents the $OH^-$ concentration from increasing significantly. ### Conclusion Because the added ions are effectively "neutralized" by the buffer components, the $pH$ of the solution remains relatively stable. Therefore, the correct option is **D** D**.