The correct answer is: A. Blood clotting
Vitamin K is a fat-soluble vitamin that is essential for blood clotting. It is also involved in bone metabolism and the production of proteins that are necessary for blood clotting. Vitamin K is found in green leafy vegetables, broccoli, Brussels sprouts, and liver.
Option B: Respiration is the process by which cells convert oxygen into energy. Vitamin K is not required for respiration.
Option C: Carbohydrate metabolism is the process by which the body breaks down carbohydrates into glucose, which is then used for energy. Vitamin K is not required for carbohydrate metabolism.
Option D: Calcium phosphorus is a mineral that is found in bones and teeth. Vitamin K is not required for calcium phosphorus metabolism.