Buffer action can be explained by:
(A) Common ion effect
(B) Law of mass action
(C) Le-Chatelier’s principle.
(D) All above.

Hint: A buffer can be defined as any solution or substance that can resist pH change of the solution in which it is present when acidic or basic solutions are added to it. It is able to neutralize the action of the acidic and basic solutions in small amounts thus maintaining the pH of the solution. The principle behind it is used to explain the changes caused by external factors to the equilibrium of a reaction.

Complete Stepwise solution
The pH of any buffer solution is calculated from the Henderson-Hasselbalch equation which is dependent upon the acid or base dissociation constant of the acid and the ratio of the conjugate acid to its base or the conjugate base to its acid and it can be mathematically expressed as:
 $ \text{pH = pKa + log }\left[ \dfrac{\text{salt}}{\text{acid}} \right] $ ,
where $ \text{Ka} $ is the acid dissociation constant. This equation can also be written as,
 $ - {\text{log}}\left[ {\text{H}} \right]{\text{ = }} - {\text{log}}\left[ {{\text{Ka}}} \right] - {\text{log}}\left[ {\dfrac{{{\text{acid}}}}{{{\text{salt}}}}} \right] $
Removing the logarithm on both sides on the equation, we get,
 $ \text{ }\left[ {{\text{H}}^{\text{+}}} \right]\text{ = Ka }\times \dfrac{\left[ \text{acid} \right]}{\left[ \text{salt} \right]} $
Or, $ \text{ Ka =}\dfrac{\left[ {{\text{H}}^{\text{+}}} \right]\left[ \text{salt} \right]}{\left[ \text{acid} \right]} $
This dissociation constant can be obtained from the Le-Chatelier’s principle which relates the concentrations of the reactants and the products with the equilibrium constant or the ionic constant.
Hence, the correct answer to this question is option C.

Note
Le-Chatelier’s principle predicts the changes that might affect the chemical equilibrium of the medium such as the effect of the temperature, the effect of the concentration of the reactants, the effect of the change in volume in case of gases, and the effect of the pressure. It explains the effect of changing these factors on equilibrium of the reaction that exists between the reactants and the products.