Answer : 4. "thrombokinase"
Explanation :

Answer: D) thrombokinase Explanation: The clotting of blood is a chain reaction initiated by the enzyme thrombokinase produced by the platelets. This causes prothrombin to be converted into thrombin in the presence of Ca++ion. Thrombin combines with fibrinogen  to produce fibrin threads which entangles the corpuscles to form the clot.