I believe that the correct answer is A. A buffer system is a chemical solution that resists any change in pH with the addition of an acid or base. A buffer solution consists of a weak acid and its conjugate base in equilibrium. An example is an acetate buffer. It consists of a solution of acetic acid and sodium acetate in water. If acid (H+) is added, the protons are neutralized by the acetate anion. If base (OH-) is added, it is neutralized by the protons from acetic acid. So the buffer is both accepting and releasing protons as needed to maintain a relatively constant pH. Option D is obviously wrong since acids and bases aren't binding to each other (they are neutralized). Option B is not correct since the buffer doesn't necessarily release hydroxide anions. C is also not correct for the same reason. Added hydroxide would be converted to water but other bases added wouldn't produce water nor would acid protons added produce water in the acetate buffer case. So A is the best choice.