The correct answer is: Cytochrome oxidase
Cytochrome oxidase is an inner membrane protein part of the electron transport chain (Complex IV). It receives an electron from each of four cytochrome c molecules, and transfers them to O2, converting it to water. Cytochrome oxidase contributes to formation of proton electrochemical potential: it binds 4 H+, and translocates another 4 H+ across the membrane, increasing the transmembrane difference of proton electrochemical potential. This gradient is used for the ATP synthesis by ATP synthase.