The correct answer is electrons.
Electron transport chain refers to an array of complexes, which mediates electrons from electron donors to electron acceptors through redox reactions, and combines the transfer of electrons with the transfer of protons through a membrane.
This produces an electrochemical proton gradient, which instigates the production of ATP. The ultimate electron acceptor in the electron transport chain is oxygen in aerobic respiration and sulfate in anaerobic respiration.
