Sodium oxalate prevents blood clotting due to

The correct option is A Combining with calcium ions of blood
Sodium oxalate, like citrates, can be used to remove calcium ions (Ca${}^2$${}^{+}$) from blood plasma. It thus prevents blood from clotting. In the presence of calcium ions and other clotting factors, factor X activates an enzyme called prothrombin activator. This enzyme then converts the plasma protein prothrombin into thrombin. Thrombin is an enzyme that, in turn, converts fibrinogen to fibrin.