What occurs in the absence of oxygen:

Explanation

In the absence of oxygen, cells can undergo anaerobic respiration, which includes processes like alcoholic fermentation and lactic acid fermentation. Both processes allow for energy production when oxygen is not available. Alcoholic fermentation involves the conversion of sugars to alcohol and carbon dioxide, primarily in yeast and some bacteria. Lactic acid fermentation involves the conversion of glucose to lactic acid, commonly occurring in muscle cells and certain bacteria. The respiratory electron transport chain, on the other hand, is an aerobic process that requires oxygen to function, making it incorrect in this context.