The correct answer is option 1, Heparin. Heparin is a natural anticoagulant produced by certain cells in our body, particularly in the liver. It works by inhibiting the formation of blood clots. When heparin is present in the blood vessel, it prevents the activation of clotting factors and slows down the clotting process. This is important to prevent clotting within the blood vessels, which can lead to blockages and serious health complications.

Options 2 and 3, Prothrombin and Haemoglobin, are not directly involved in preventing clotting in the blood. Prothrombin is actually a clotting factor that participates in the formation of blood clots rather than inhibiting it. Haemoglobin, on the other hand, is responsible for carrying oxygen in red blood cells and does not have a direct impact on clotting.

Option 4, Hirudin, is an anticoagulant that is secreted by leeches. Although it is similar to heparin in function, it is not naturally present in the human body. Therefore, while hirudin can prevent clotting, it is not the main factor responsible for the failure of blood to clot in blood vessels.

Overall,