The correct answer is option 3: Heparin. Heparin is a natural anticoagulant produced by the body. It works by inhibiting the action of thrombin, which is an enzyme involved in the clotting process. Thrombin plays a crucial role in converting fibrinogen into fibrin, which forms a clot.

Option 1: Thrombin is involved in the clotting process, so it does not prevent clotting.

Option 2: Fibrinogen is a protein that is converted to fibrin by thrombin, contributing to clot formation.

Option 4: Prothrombin is a precursor to thrombin and does not prevent clotting.

Therefore, it is clear that the presence of heparin is what prevents blood from clotting in the blood vessels.