The correct option is A prothrombin
Blood coagulation is a very vital process during injuries, where the fibrins form a clot or coagulam in order to stop the process of bleeding.
In the presence of the enzyme thrombokinase, an inactive substance, prothrombin is converted to an active enzyme, thrombin. This, now, converts the inactive fibrinogen into active fibrin. This fibrin forms a mesh-like structure at the site of injury and traps the blood particles. This results in the clotting of the blood.