Which of these is not a product of fermentation?

The correct option is Oxygen.

Fermentation is an anaerobic metabolic pathway, meaning it occurs in the absence of oxygen. During this process, cells break down glucose (or other nutrients) to generate energy in the form of adenosine triphosphate (ATP) without using an electron transport chain or oxygen as a final electron acceptor.

Depending on the type of organism and the cellular environment, different products are formed during fermentation:
1. Lactic Acid Fermentation: In organisms like bacteria (e.g., Lactobacillus) and animal muscle cells under low-oxygen conditions, pyruvate is reduced to lactate.
2. Alcohol Fermentation: In yeast and some bacteria, pyruvate is converted into ethanol and carbon dioxide.

Since oxygen is not produced during any of these pathways—and is, in fact, the very substance whose absence defines anaerobic processes like fermentation—it is not a product of fermentation. Instead, oxygen is a reactant in aerobic cellular respiration.