The correct option is D Potassium ions
Fibrins, thrombins, prothrombin, thrombokinase and calcium ions are all equired for blood clotting.
Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin.
Thrombins, in turn are formed from another inactive substance present in the plasma called prothrombin.
An enzyme complex, thrombokinase, is required for the above reaction which is formed by a series of linked enzymic reactions (cascade process) involving a number of factors present in the plasma in an inactive state.
Calcium ions play a very important role in clotting.