Which factor is crucial for the synthesis of clotting factors?

Vitamin K is essential for the synthesis of several key clotting factors in the body, specifically factors II (prothrombin), VII, IX, and X. These factors are vital components in the coagulation cascade, which is the process that leads to the formation of blood clots to stop bleeding. Vitamin K functions as a cofactor for the enzyme gamma-glutamyl carboxylase, which is responsible for post-translational modifications of these factors. This modification is critical for their biological activity, allowing them to bind calcium ions, which is necessary for the clotting process to occur.

In contrast, while vitamins A, C, and D play important roles in various physiological processes, they do not directly contribute to the synthesis of clotting factors. Vitamin A is primarily associated with vision and immune function, vitamin C is involved in collagen synthesis and is an antioxidant, and vitamin D is critical for bone health and calcium regulation. Therefore, the correct choice identifies the specific vitamin directly linked to the production and function of clotting factors.