The correct answer is option 2: enzymes.

Fermentation is a biological process where microorganisms, such as bacteria or yeast, break down organic compounds, such as sugars or carbohydrates, into simpler substances like alcohol or acids. Enzymes are proteins that act as catalysts in biochemical reactions, including fermentation. They speed up the process of breaking down organic compounds without being consumed in the process. Enzymes work by binding to specific substrates, in this case, the organic compounds, and causing a chemical reaction to occur, resulting in the decomposition of the compounds. Catalysts (option 1), carbanions (option 3), and free radicals (option 4) are not directly involved in the process of fermentation. Therefore, option 2, enzymes, is the correct answer.