Which of these substances acts as a buffer?

a. blood

b. lemon juice

c. curd

The correct answer is (A) blood.

when the buffer solution its PH value changes very little when a small amount

of strong acid or base is added to it, and here the bicarbonate buffering system is used to regular the PH of the blood that keeping the PH at nearly constant value by maintaining the original acidity or basicity of the solution.