The process that does not require oxygen is fermentation.
Explanation:
Fermentation is an alternative route of cellular respiration for obtaining energy, in the absence of oxygen. In aerobic respiration, oxygen is the acceptor of electrons from NADH but, in anaerobic respiration, the electron acceptor is an organic compound.
The fermentation process does not occur in the mitochondria nor does it involve the electron transport chain, and has a low energy yield at the end of the process.
With respect to the other options:
<em>The </em><em><u>electron transport chain</u></em><em> and the </em><em><u>Krebs cycle</u></em><em> occur in the presence of oxygen.</em>
