The correct answer is fermentation.
The production of energy needs oxygen. The electron transport chain, where the most of ATP is produced, needs a huge input of oxygen. However, there are various species, which have produced methods to continue the metabolism in the absence of oxygen, or can move from aerobic to anaerobic cell respiration when oxygen is enough.
At the time of cellular respiration, some of the living species utilize an organic substance as the eventual electron acceptor. The procedures that utilize an organic molecule to reproduce NAD+ from NADH are in a combination known as fermentation.
In comparison, some of the living species utilize an inorganic substance as an ultimate electron acceptor. Both the procedures are known as anaerobic cellular respiration, where the species are obtaining energy for their application in the absence of oxygen.