The function of 'buffer solution' is:
A. to increase the pH value in chemical reaction
B. to decrease the pH value on chemical reaction
C. to increase or decrease the pH value of according the need
D. to keep the pH value constant in chemical reaction

Hint: A buffer solution also known as pH buffer or hydrogen ion buffer is an aqueous solution which consists of a mixture of a weak acid and its conjugate base or vice versa.

Complete step by step solution:
The term buffer solution is used to describe the solution mixture of a weak acid and its conjugate base. The main purpose of buffer solution is to resist the change in pH value of a solution upon adding a small amount of acid and base. pH of buffer solution remains constant because they contain an acid component say HA, to neutralize $\text{O}{{\text{H}}^{\text{-}}}$ ions and a base component, ${{\text{A}}^{\text{-}}}$ to neutralize ${{\text{H}}^{\text{+}}}$ ions. Therefore, we can conclude that the main function of 'buffer solution' is to keep the pH value constant in chemical reaction. So, option D is correct.
Additional Information:
In Industry, buffering agents are used in fermentation processes and in setting the correct conditions for dyes which are used in coloring fabrics.
They can also be used in chemical analysis and calibration of pH meters.

Note: Generally we have the misconception that the pH of buffer solution can't change. No, it's not like that, pH of buffer solution changes but it can only vary within a narrow range, regardless of what else may be present in the solution.