By adding a strong acid to the buffer solution, the pH of the buffer solution
(A) Remain constant
(B) Increases
(C) Decreases
(D) Becomes zero

Hint: Buffer solutions maintain the pH of any solution. There is a small amount of change in pH. In addition to strong acid in a buffer solution the pH will get affected slightly. A strong acid will dissociate and release H+ ions and as the H+ concentration increases the pH decreases slightly.

Complete answer:
Buffer solutions are those mixtures that resist the change in pH of a solution. It consists of a mixture of a weak acid and its conjugate base or it can also be a weak base and its conjugate acid. They do not allow the pH of any solution to change. The buffer solutions are able to maintain the hydrogen ion concentration (pH) with only minor changes on the dilution or addition of a small amount of either base or an acid. They are used in food preservatives, fermentation, chemical reactions,s etc.
Buffer solutions can be acidic and basic. Acidic buffers are those which maintain the acidic environment of the solution. Basic buffers maintain the basic environment of the solution.
Now let us see what happens to the buffer solution when we add strong acid to it.
When a strong acid is added to a buffer solution the conjugate base that is present in the buffer consumes the H+ ions. This gets converted into water and the weak acid of the conjugate base. This can be understood by the following equation:
 $ {A^ - }_{{\text{(aq)}}} + {H_3}{O^ + }_{{\text{(aq)}}} \to {H_2}{O_{{\text{(l)}}}} + H{A_{{\text{(aq)}}}} $
Now when this happens there is a decrease in the amount of conjugate base present and an increase in the amount of the weak acid. This leads to the decrease in the pH of the buffer solution. This change in pH is very small.
Therefore adding a strong acid to the buffer solution, the pH of the buffer solution decreases.
Hence the correct option is C; Decreases.

Note:
a buffer solution has to contain things that will remove any H+ ions or OH- ions that might be added to it. Otherwise the pH will change acidic and basic buffer solutions achieve this in different ways. There are many examples of buffer solution-blood is the most common example which contains a bicarbonate buffer system.