Explanation:

1. Fermentation:
Fermentation is the energy-releasing metabolic process in which the substrate is oxidized without the presence of an external electron acceptor. It is an anaerobic process that occurs in the absence of oxygen. During fermentation, the substrate molecules undergo partial oxidation, resulting in the production of energy in the form of ATP.

2. Glycolysis:
Glycolysis is the metabolic pathway that occurs in the cytoplasm of cells and is the initial step in both aerobic respiration and fermentation. It involves the breakdown of glucose into two molecules of pyruvate. While glycolysis is a common step in both fermentation and aerobic respiration, it does not directly involve the oxidation of the substrate without an external electron acceptor.

3. Photorespiration:
Photorespiration is a process that occurs in plants when there is a shortage of carbon dioxide and excess oxygen. It involves the oxidation of carbohydrates in the presence of light, resulting in the release of carbon dioxide. Photorespiration is not related to the oxidation of substrate without an external electron acceptor.

4. Aerobic Respiration:
Aerobic respiration is the energy-releasing metabolic process that occurs in the presence of oxygen. It involves the complete oxidation of the substrate, such as glucose, to produce energy in the form of ATP. During aerobic respiration, the substrate is oxidized with the help of an external electron acceptor, which is usually oxygen.

Conclusion:
Among the given options, fermentation is the correct answer because it involves the oxidation of substrate without the requirement of an external electron acceptor. Fermentation is an anaerobic process and occurs in the absence of oxygen. It is a less efficient process compared to aerobic respiration, but it allows cells to produce ATP when oxygen is not available.