Haemoglobin is the Quaternary structure of the protein and it is shown by proteins that contain more than one polypeptide chain. When the units are dissimilar, it is known as heterogeneous Quaternary change. As in the case of haemoglobin, it is made up of two subunits as 2 alpha and 2 beta subunits. They all four are combined along with the iron molecule that carries the oxygen. One haemoglobin carries four oxygen molecules. Thus, the correct answer is '4 subunits (2α, 2β).'