Correct answer: (2) Requires an electron transport system.

Fermentation, in biochemical terms, is an anaerobic process where ATP is generated by substrate-level phosphorylation and an organic compound acts as the final electron acceptor; it does not use an electron transport chain.

Option analysis:

• (1) Alcohol is formed from sugar residues
  This is true for alcoholic fermentation, where sugars are converted to ethanol and CO₂. It is a valid example of fermentation, so it can be included in defining specific types of fermentation.

• (2) Requires an electron transport system
  This is NOT true for fermentation. Fermentation does not involve an electron transport chain; instead, NADH is reoxidized by transferring electrons to organic intermediates. This statement cannot be included in the definition, so this is the correct choice.

• (3) Utilizes an organic compound as the final electron acceptor
  This is a core feature of fermentation: pyruvate or its derivatives (organic molecules) act as final electron acceptors, not oxygen or other inorganic acceptors. So this belongs in the definition.

• (4) Produces lactic acid in oxygen deprived muscle
  This describes lactic acid fermentation in muscle cells under anaerobic conditions, a classic example of fermentation. It can be included in defining fermentation with examples.