Hint: In humans when it is a cut or an injury on the skin then the blood comes out of that cut. This blood gets stopped on its own by a mechanism of blood clotting. This clot helps in the stoppage of the overflowing of blood from the cut or injury.
Blood clotting is a necessary process as it helps in preventing the excess flow of the blood when there is an injury in a person. Thrombin is the blood-clotting enzyme which helps in the conversion of fibrinogen to fibrin. This enzyme also helps in the formation of the clot which is formed by the fibrin. This enzyme is produced in the liver. It helps in the aggregating platelet which causes the clot of the blood. Other than the enzyme prothrombin, vitamin K is also helpful in clotting the blood. It also helps in healing the wound. The prothrombin is present in the inactive form. It gets activated to form the thrombin enzyme. Prothrombin is the vitamin K dependent coagulation unit. The calcium ions are also used as the blood clotting factors. There can be mutations in the prothrombin and can lead to severe disease. It causes thrombophilia in humans.
So, the correct answer is ‘liver’.
Note: The prothrombin is present in the inactive form in the bloodstream. It is formed in the liver. When there is an injury or cut in the body of the human beings the prothrombin changes itself into its active form known as thrombin. Thrombin then helps in the coagulation of the blood.