Answer:
Oxygen
Explanation:
In electron transport chain there are a series of complexes involved which facilitate the transfer of electrons from electron donors to electron acceptors and this electron transport is coupled with the transfer of Hydrogen ions across the membrane. The accumulation of hydrogen ions creates a proton gradient across the membrane and further ATP is synthesized with the help of this proton gradient.
There are four complexes labelled as complex I, II, III, and IV are involved in the electron transport chain. The complex IV keeps and oxygen molecule in reduced form. This reduced oxygen acts as the final acceptor of hydrogen ions and picks up two hydrogen ions to form a water molecule. The removal of hydrogen ions contributes to the ion gradient across the membrane.