A.Correct option - A
B.Explanation for correct option -A
The oxyhaemoglobin dissociation curve describes the non-linear tendency for oxygen to bind to haemoglobin. 
The oxyhaemoglobin dissociation curve mathematically calculates and determines the percentage saturation of haemoglobin to the partial pressure of oxygen in the blood.
The strange sigmoid shape of the curve relates to peculiar properties of the haemoglobin molecule.