Answer:
The correct answer would be NADH and FADH₂.
Electron transport system (ETS) or electron transport chain (ETC) refers to the series of complexes present in the inner membrane of the mitochondria.
These complexes act as electron carriers which transfer high energy electrons provided by NADH and FADH₂ to oxygen to form water.
When these high energy electrons travel through ETC they loose energy some of which is used by these complexes to pump hydrogen ions into the inter-membrane space of the mitochondria.
It leads to the formation of proton gradient across the inner membrane which is used by the ATP synthase (also located in inner membrane) to phosphorylate ADP into ATP.