Buffers are most effective at resisting changes in pH when—
a) the pH is about one unit above the acid’s pKa, so that [A−]≈10[HA].
b) the pH equals the acid’s pKa and the concentrations of the acid and its conjugate base are nearly the same.
c) the pH is far below the acid’s pKa, but both the acid and its conjugate base are present at very high concentrations.
d) the pH equals the acid’s pKa, but the concentration of the acid is much greater than that of its conjugate base.