Final answer:
The principal site for the synthesis of vitamin K by microorganisms is the human intestine. Bacteria present in the large intestine help in the production of vitamin K, which is necessary for blood clotting and prothrombin formation.
Step-by-step explanation:
The principal site for the synthesis of vitamin K by microorganisms is the human intestine. In the human gut, bacteria present in the large intestine produce vitamin K. These bacteria help in the breakdown of food and aid in the production of valuable nutrients, including vitamin K. Additionally, vitamin K is necessary for proper blood clotting and is involved in the formation of prothrombin.