The correct answer is C, ATP.
ETS is an acronym for electron transport chain. Electron transport chain refers to the series of reactions which occurs during cellular respiration in which energy is transferred from the electrons and protons carried by FADHD2 and NADH in order to generate ATP molecules. The reactions of ETS is a series of oxidation reduction reaction which are geared toward generating ATP molecules, that is, ATP is the product of the reactions.