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Buffer solutions have constant acidity and alkalinity because:
A
they have fixed value of pH
B
these give unionised acid or base on reaction with added acid or alkali
C
acids and alkalies in these solutions are shielded from attack by other ions
D
they have large excess of $H^+$ or $OH^-$ ions
Detailed Solution
A buffer contains a weak acid and its conjugate base (or a weak base and its conjugate acid).
Added $H^+$ combines with the conjugate base to form the unionised weak acid, e.g. $CH_3COO^- + H^+ \rightarrow CH_3COOH$.
Added $OH^-$ reacts with the weak acid: $CH_3COOH + OH^- \rightarrow CH_3COO^- + H_2O$.
Since the added acid or alkali is converted into unionised acid or base, the pH stays almost constant.
Added $H^+$ combines with the conjugate base to form the unionised weak acid, e.g. $CH_3COO^- + H^+ \rightarrow CH_3COOH$.
Added $OH^-$ reacts with the weak acid: $CH_3COOH + OH^- \rightarrow CH_3COO^- + H_2O$.
Since the added acid or alkali is converted into unionised acid or base, the pH stays almost constant.