Which of the following substances, if introduced in the bloodstream, would cause coagulation, at the site of its introduction?

Thromboplastin is a substance that, if introduced into the bloodstream, would cause coagulation at the site of its introduction. Thromboplastin is a protein that is involved in the coagulation cascade, and it plays a critical role in the formation of blood clots. So, the correct answer is option D. Notice that the other options are not correct. Fibrinogen is a protein that is involved in the coagulation cascade, but it is not the substance that would cause coagulation if introduced into the bloodstream. Prothrombin is a protein that is involved in the coagulation cascade, but it is not the substance that would cause coagulation if introduced into the bloodstream. Heparin is an anticoagulant that would actually prevent coagulation if introduced into the bloodstream.