The mechanism of blood clotting involves the presence of calcium and other clotting factors, Factor X activates an enzyme called prothrombin activator. The enzyme prothrombin activator then converts plasma protein prothrombin into thrombin. Thrombin is the enzyme which in turn converts fibrinogen into fibrin. Polymerized fibrin together with platelets forms a clot at the wound site. Hence Protein required for coagulation of blood is Fibrinogen. It is a blood plasma protein made in the liver and present in circulating blood. So, the correct answer is 'Fibrinogen'

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