a
If a small amount of an acid or alkali is added to a buffer solution, it converts them into unionised acid or base. Thus, remains unaffected or in other words its acidity/alkalinity remains constant. e.g.

\(\mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{A}^{-} \rightleftharpoons \mathrm{H}_{2} \mathrm{O}+\mathrm{HA}\)

\(\mathrm{OH^-}+\mathrm{HA} \rightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{A}^{-}\)

If acid is added, it reacts with \(A^-\) to form undissociated \(HA\). Similarly, if base/alkali is added, \(OH^-\) combines with \(HA\) to give \(H_2O\) and Aand thus, maintains the acidity/alkalinity of buffer solution.