The correct answer is option 2, Heparin. Heparin is an anticoagulant substance that is naturally present in the body. It is produced by cells in the lining of blood vessels and helps to prevent blood from clotting too quickly. Heparin works by inhibiting the action of several clotting factors, including thrombin and fibrinogen. Thrombin, on the other hand, is not an anticoagulant but a clotting factor that helps convert fibrinogen to fibrin, which is necessary for blood clot formation. Fibrinogen is a protein involved in blood clotting but does not have anticoagulant properties. Globin, lastly, refers to a type of protein found in hemoglobin, a molecule responsible for carrying oxygen in red blood cells and is not involved in blood clotting or anticoagulation.