Which inner membrane protein directly catalyzes the reduction of o2 to h2o?

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.