Which of the following ion is needed for the conversion of prothrombin to thrombin?

Prothrombin activator is formed as a result of rupture of a blood vessel or as a result of damage to special substances in the blood. Then the prothrombin activator, in the presence of sufficient amounts of ionic Ca++, causes conversion of prothrombin to thrombin. Thrombin causes polymerization of fibrinogen molecules into fibrin fibers. Ref: Physiology Secrets By Hershel Raff, 2003, Page 66; Guyton's physiology 22nd edition, Page 459.