Prothrombin is related to (b) Blood coagulation. It is a vital component in the process of clot formation, known as hemostasis. When blood vessels are injured, prothrombin is converted into thrombin through a series of enzymatic reactions, leading to the formation of a fibrin clot. This clotting mechanism helps to prevent excessive bleeding by sealing off damaged blood vessels. Without prothrombin and the subsequent clotting process, individuals would be at risk of hemorrhage even from minor injuries, highlighting the critical role of prothrombin in maintaining vascular integrity and overall hemostasis.