The answer is a.blood.
The role of the buffer solution is to maintain pH in a solution. In experiments it is used to changes of pH in the solution. It does this because it consists of a weak acid and conjugated base or vice versa.
In blood, bicarbonate buffer system is present. It maintains pH of the blood, by maintaining the balance of carbon dioxide, bicarbonate ion, and carbonic acid. Because of this buffer system in blood, blood could act as a buffer.
