Which of the following substances is regenerated in the electron transport chain?

In the electron transport chain (ETC), NAD+ is indeed regenerated. This process occurs in the mitochondria during cellular respiration, particularly in the oxidative phosphorylation stage. As electrons are transferred through a series of protein complexes, NADH, which carries high-energy electrons, is oxidized back to NAD+.

This regeneration is crucial because it allows NAD+ to be available for glycolysis and the citric acid cycle, where it plays an essential role as an electron carrier. Without NAD+, these metabolic pathways would be disrupted, leading to a decrease in ATP production.

In contrast, NADH and FADH2 serve as electron donors in the ETC, while ATP is produced as a result of the proton gradient established during the electron transfer process, but it is not regenerated in the same manner as NAD+. This clarification emphasizes the functional role of NAD+ in maintaining the flow of metabolic processes and energy production within the cell.