The correct answer is option 2, Heparin. Heparin is a natural anticoagulant that is produced by the liver and endothelial cells. It prevents blood from coagulating by inhibiting the activity of several clotting factors, specifically thrombin. Thrombin is a key enzyme in the blood clotting process, and its inhibition by heparin prevents the formation of fibrin, which is necessary for blood clot formation.

Option 1, Haemoglobin, is the oxygen-carrying molecule in red blood cells. While it plays a critical role in transporting oxygen, it is not involved in preventing blood coagulation.

Option 3, Fibrin, is a protein that forms a mesh-like structure and is responsible for the formation of blood clots. It is not involved in preventing clotting but is the end result of the coagulation process.

Option 4, Plasma, is the liquid component of blood that contains various proteins, including clotting factors. While plasma does contain some components involved in blood clotting, it does not actively prevent clot formation like heparin does.

Therefore, the presence of heparin in the body is what prevents blood from coagulating. Alert - Correct answer should be 3 (