Correct Option: B

Because the process of blood clotting starts when prothrombinase (an enzyme formed by thromboplastin) catalyzes the conversion of prothrombin (an inactive protein in the blood plasma) into thrombin (active protein) and this reaction takes place in the presence of Ca²⁺. Now this thrombin converts fibrinogen into fibrin which makes fibrin threads on polymerization & with blood corpuscles, it clots blood, som if Ca²⁺ are removed from blood it’ll prevent clotting. It may also be done by adding some anti coagulants eg. sodium & potassium oxalates in blood as they precipitate Ca2+ present in the blood.