The correct answer is Chelate calcium ions

Oxalates and citrates are commonly used as anticoagulants in stored blood because they chelate calcium ions, which are essential for the coagulation (clotting) of blood.

Option 1, “Chelate calcium ions”, is the correct answer. Calcium ions play an important role in the coagulation cascade, a complex process that leads to the formation of blood clots. By chelating calcium ions, oxalates and citrates prevent the coagulation cascade from being activated, thereby preventing the formation of blood clots in stored blood.

Option 2, “Act as competitive inhibitors of vitamin K”, is not correct. Vitamin K plays a role in the synthesis of several coagulation factors, but oxalates and citrates do not affect vitamin K function.

Option 3, “Potentiate anti-thrombin III”, is also not correct. Antithrombin III is a natural anticoagulant protein in the blood, but oxalates and citrates do not potentiate its activity.

Option 4, “Activate heparin”, is also not correct. Heparin is another commonly used anticoagulant, but oxalates and citrates do not activate its activity.

In conclusion, option 1, chelating calcium ions, is the correct answer as to why oxalates and citrates are used as anticoagulants in stored blood.