Ans. is 'b' i.e., Complex IIInner mitrochondrial membrane is impermeable to protons; therefore, proton pumps are required to pump protons (hydrogen ions) from mitochondrial matrix to the intermembrane space.The complex I (Site I), complex III (Site II) and complex IV (Site III) act as proton pumps ejecting protons (hydrogen ions) from the mitochondrial matrix to intermembrane space.Complex I (NADH-CoQ reductase) and complex III (CoQ - cytochrome c reductase) pump 4 protons each and complex IV (cytochrome c oxidase) pumps 2 protons.The concentration of hydrogen ions (protons) on the outer side of inner membrane becomes higher as compared to inner side.This results in generation of the electrochemical potential.Due to this electrochemical potential or proton motive force, the H+ ions ejected out (by proton pumps) flow back into the mitochondrial matrix down its electrochemical gradient through F0F1 ATPase molecule, which is also known as complex V of respiratory chain.This proton influx causes ATP synthesis. Fo components acts as channel for passage of hydrogen ions (Protons). F1 component possesses ATP synthase activity, which is switched on when the hydrogen ions pass through Fo component.