Explanation

Vitamin K is essential for blood clotting because it enables the gamma-carboxylation of clotting factors II (prothrombin), VII, IX and X in the liver, without which they cannot bind calcium and function. Its very name comes from the Danish 'koagulation'. Vitamins A, B and D have no direct role in the coagulation cascade: A concerns vision, the B group concerns metabolism, and D concerns calcium homeostasis. The clotting vitamin is therefore K, option (D).