Organisms that live and reproduce in the absence of oxygen

Those organisms who live and reproduce in the absence of oxygen are termed as anaerobes. Those anaerobes that can not manage the presence of oxygen and harmed by even small quantity of oxygen are called obligate anaerobe, for example, Clostridium botulinum.

Those organisms which have the ability to grow in the absence of oxygen but utilize oxygen when oxygen is present are called facultative anaerobes for example in human intestine E.coli and other bacteria of Enterobacteriaceae family are facultative anaerobe.

There is one other category that contains anaerobe and called aerotolerant organisms which grow in the absence of oxygen and are tolerant towards oxygen. Therefore the correct answer is anaerobes.