The correct option is A A proton gradient forms across the inner membrane
Chemiosmotic theory was postulated by the British biochemical Peter Mitchell (1920-1922) to explain the formation of ATP in the mitochondrial electron transport chain.
As electrons are transferred along the electron carrier system in the inner mitochondrial membrane, hydrogen ions (protons) are actively transported into the space between the inner and outer mitochondrial membranes.
Thus an electrochemical gradient is created across the inner membrane, which is utilised by the enzyme ATP synthase to generate ATP.