The correct answer is option 3, Heparin. Heparin is a natural anticoagulant that is produced by certain cells in the body. Its main function is to prevent blood from clotting in the blood vessels. Heparin works by inhibiting the activity of thrombin, an enzyme that is essential for the formation of blood clots. By preventing the formation of fibrin, which is necessary for blood clotting, heparin ensures that the blood remains in a liquid state and does not coagulate inside the body. Option 1, Plasma, is incorrect because plasma is the liquid component of blood but it does not directly prevent blood clotting. Option 2, Haemoglobin, is incorrect because while it carries oxygen in the blood, it does not have a role in preventing blood clotting. Option 4, Fibrin, is incorrect because fibrin is actually a product of blood clotting and not a factor that prevents it.