The correct answer is a colon.
The bacterial flora of the gastrointestinal tract is very diverse, especially in the large intestine (colon) which has the richest microflora. The main types of microorganisms in the colon are obligate anaerobes, and the most abundant bacteria are Bacteroides. Functions of the microflora are useful for hosts such as protective function, prevention of growth of harmful species, regulation of the development of the gut, production of vitamins for the host (vitamin K) and hormone producing.