A_ is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent.

Explanation

Facultative anaerobes are bacteria that can grow in both the presence or absence of oxygen. In addition to oxygen concentration, the oxygen reduction potential of the growth medium influences bacterial growth. The oxygen reduction potential is a relative measure of the oxidizing or reducing capacity. Obligate anaerobes are those that carry out anaerobic respiration only and can not carry out aerobic respiration even in presence of oxygen. Cellular anaerobes and respiratory anaerobes are terms not used to designate any type of organism.