E.coli is the microflora of the human body which synthesizes vitamin B and K in the intestine. They live upon the undigested food passing through the caecum and help in better absorption of food and prevent the growth of pathogenic bacteria in the intestine. These vitamins, except B12, are water-soluble and cannot be stored in the body for a long time, making E.coli more important.So, the correct answer is option D.