# The most important buffer in the blood consists of:(a)- $HCl\text{ and C}{{\text{l}}^{-}}$(b)- ${{H}_{2}}C{{O}_{3}}\text{ and HC}{{\text{O}}_{3}}^{-}$(c)- ${{H}_{2}}C{{O}_{3}}\text{ }and\text{ C}{{\text{l}}^{-}}$(d)- $HCl\text{ and HC}{{\text{O}}_{3}}^{-}$

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Hint:
Buffer solution is a solution in which a small amount of acid or base would change the pH of the solution. The buffer of the blood contains a bicarbonate ion. It also has carbon dioxide.

Complete step by step answer:
First, let us understand the buffer solution:
-It is sometimes necessary that a solution of definite pH be prepared and stored. The preservation of these kinds of solutions are very difficult, even more difficult than their preparation.
-Hence, a buffer solution is defined as a solution that resists any change in its pH value ( i.e., whose pH remains practically constant) even when a small amount of the acid or the base is added to it.
-There are two types of the buffer:
The solution of single substances: The solution of a salt of a weak acid and weak base. For example, ammonium acetate acts as a buffer.
The solution of mixture: It is of two types: acidic buffer and basic buffer.
-The pH of our blood is maintained constant (at about 7.4) despite various acid and base producing reactions going on in our body. In the absence of its buffer nature, we could not eat a variety of foods and spices.
-The buffer action is due to the presence of carbonic acid (${{H}_{2}}C{{O}_{3}}$), bicarbonate ion ($\text{HC}{{\text{O}}_{3}}^{-}$), and carbon dioxide ($C{{O}_{2}}$) in the blood.

Hence, the correct answer is option (b)- ${{H}_{2}}C{{O}_{3}}\text{ and HC}{{\text{O}}_{3}}^{-}$.

Note:
HCl is a strong acid and hence do not use in buffer solutions.The buffer solution is used in many applications like electroplating, manufacture of leather, preservation of food, etc.