The correct answer is option 1: Vitamin K. Intestinal bacteria in the human body are responsible for synthesizing Vitamin K. Vitamin K is a fat-soluble vitamin that plays a key role in blood clotting and bone health. While all the options listed can be found in the human body, only Vitamin K is specifically synthesized by intestinal bacteria. Vitamin K is not produced by the human body itself, so we rely on the bacteria in our intestines to produce it for us. This is why maintaining a healthy balance of intestinal bacteria through a balanced diet, including foods rich in fiber, can help ensure adequate Vitamin K production. It is important to note that this process occurs in the large intestine, where the intestinal bacteria reside.