Answer: Oxygen transportation in blood takes place by the Heamoglibin.
Explanation: Heamoglibin is an protein molecule with an iron in it. It is present in red blood cell and is an oxygen transporting molecule.
Each Heamoglibin molecule bound to the four oxygen molecules and form an complex which is known as oxyhemoglobin.
With this Oxyhemoglobin complex oxygen is transported into the blood.