It has been estimated that 99% of the intestinal bacteria are a. facultative anaerobes. b. obligate aerobes. c. facultative aerobes. d. obligate anaerobes.

Obligate anaerobes. (Ans. D)

Explanation:

Anaerobes: They are micro-organisms that do not require energy oxygen for metabolism. They can only survive in an environment that lacks oxygen.

Obligate anaerobes are bacteria typically living in various places naturally. Many of obligate anaerobes living in the human body, in places such as the mouth and gastrointestinal tract (most of the obligate anaerobes present their) where oxygen level present in very low concentration.

Obligate anaerobes do not contain several enzymes needed to detoxifying oxygen present in the cell. Oxygen produces several byproducts in the presence of water such as hydrogen peroxide, it is a week acid and affects no. of enzymes present in anaerobic cells. In a high level of oxygen anaerobic cells become more acidic and not able to function.

So that's why they are found in no oxygen or low oxygen environment like the gastrointestinal tract.  

condensation. good luck!