The correct option is C (a) and (b) above.
Human blood contains a buffer of Carbonic acid (H2CO3) and bicarbonate anion (HCO−3). Whenever any acidic substance enters the blood, the bicarbonate ions neutralize the hydronium ions forming carbonic acid and water. Thus hydronium ions are removed preventing the pH of blood from becoming acid.
River water prevents localized change in pH as a result of which no change may be measured under natural condition.
Hence both blood and river water are natural buffer.