The correct answer is option 3, Vitamin K. Vitamin K is essential for the coagulation of blood. Let`s go through each option to understand why the other options are not correct choices.

Option 1, Vitamin A, is not essential for the coagulation of blood. Vitamin A plays a role in promoting healthy vision, bone growth, and immune system function, but it does not directly affect blood clotting.

Option 2, Vitamin B12, is also not essential for blood coagulation. Vitamin B12 is involved in the production of red blood cells and the maintenance of the nervous system, but it does not have a direct impact on blood clotting.

Option 4, Vitamin D, is important for bone health and helps with the absorption of calcium, but it does not play a role in blood coagulation.

Therefore, the correct answer is option 3, Vitamin K. Vitamin K is necessary for the process of coagulation, or clotting, of blood. Without it, blood would not be able to clot properly, leading to excessive bleeding.