What Role Do Hydrogen Ions (H⁺) Play in the Electron

What role do hydrogen ions (H⁺) play in the electron transport chain?

A) They are pumped across the mitochondrial inner membrane against their concentration gradient (to where their concentration is high) ; as the H⁺ ions flow back to where their concentration is low, they drive ATP synthase to form ATP.
B) They are pumped across the mitochondrial inner membrane against their concentration gradient (to where their concentration is low) ; as the H⁺ ions flow back to where their concentration is high, they drive ATP synthase to form ATP.
C) They add phosphate groups on ADP to form ATP.
D) They transfer electrons to the electron transport chain.