The substance that causes the clotting of blood is:

(A) Thrombin

Thrombin is an enzyme that plays a crucial role in the blood clotting process. It is responsible for converting fibrinogen, a soluble plasma protein, into insoluble fibrin threads. These fibrin threads form a mesh that helps in the clotting of blood, particularly at the site of injury or wound. Therefore, option (A) Thrombin is the correct answer.