Final answer:
Vitamin K is essential for blood-clotting as it plays a critical role in the formation of prothrombin and the blood clotting process, with its deficiency leading to serious bleeding issues.
Step-by-step explanation:
The vitamin that is essential for blood-clotting functions is vitamin K. Vitamin K is necessary for the proper formation of prothrombin, a blood plasma protein and precursor of thrombin. Thrombin is an enzyme that converts fibrinogen into fibrin, which is the key protein that holds blood clots together. Vitamin K not only plays an integral role in blood clotting by increasing the activity of clotting factors and initiating the biosynthesis of proconvertin in liver cells, but it also acts as a coenzyme for carboxylation processes and participates in the electron transport chain. A deficiency in vitamin K can lead to increased clotting time, decreased blood prothrombin levels, and continuous bleeding, especially during childbirth.