The finger like projections of intestine known as villi increases surface area of small intestines. The surface of small intestine is lined up with epithelial cells containing brush borders or microvilli that help to absorb nutrients into the blood or lymph, which later moves to the body cells. Due to the presence of specialized proteins within the microvilli known as porters, the nutrients move into the cells. Human gut flora contains several types of bacterial species in the small intestines that are involved in the production of vitamin K, B as well as metabolism of bile acid and xenobiotics. So, correct option is d.transferring nutrients across intestinal membranes Solution The finger like projections of intestine known as villi increases surface area of small intestines. The surface of small intestine is lined up with epithelial cells containing brush borders or microvilli that help to absorb nutrients into the blood or lymph, which later moves to the body cells. Due to the presence of specialized proteins within the microvilli known as porters, the nutrients move into the cells. Human gut flora contains several types of bacterial species in the small intestines that are involved in the production of vitamin K, B as well as metabolism of bile acid and xenobiotics. So, correct option is d.transferring nutrients across intestinal membranes.