# The greatest affinity for haemoglobin is shown by

$(a)\;NO\qquad(b)\;CO\qquad(c)\;O_2\qquad(d)\;CO_2$

Hemoglobin in humans has a very high affinity for carbon monoxide, forming carboxyhemoglobin which is a very bright red in color. Carbon monoxide is thus problematic for humans because it has affinity higher than that of oxygen.
The greatest affinity for haemoglobin is shown by CO
Hence (b) is the correct answer.
edited Feb 26