The correct option is A Inner membrane of mitochondria
Oxidative phosphorylation refers to synthesis of ATP by ADP and Pi driven by energy released from oxidation of reducing powers NADH and FAD by transfer of their electrons through electron trasnport chain to oxygen. Electron trasnport chain is present in inner mitochondrial membrane and passes the electrons down a series of carriers and pump protons into the intermembrane space. Oxygen serves as final electron acceptor and the resulting proton gradient serves as energy source for the enzyme ATP synthase to synthesize ATP. Correct answer is B.