Final answer:
The process where substrate is oxidized without an external electron acceptor is called fermentation, which allows glycolysis to continue producing ATP in anaerobic conditions.
Step-by-step explanation:
The energy-releasing metabolic process in which substrate is oxidized without an external electron acceptor is called fermentation. Unlike aerobic respiration, which requires oxygen, fermentation can occur in anaerobic conditions, meaning in the absence of oxygen.
Glycolysis is the first step in both aerobic respiration and fermentation, where glucose is broken down to pyruvate, but in fermentation, the pyruvate is used to regenerate NAD+ from NADH, allowing glycolysis to continue and produce energy in the form of ATP. This regeneration of NAD+ does not result in the production of additional ATP beyond that produced by substrate-level phosphorylation during glycolysis. Hence, the correct answer to the question is A. Fermentation.