The correct option is C Electron transfer
Cytochromes are proteins with heme prosthetic groups and characteristic strong absorption of visible light. The iron component of cytochromes accepts electrons from hydrogen atoms followed by transfer of accepted electrons to some other compound thereby serving as electron transfer agents. For example, mitochondria contain cytochrome a, b, and c as a component of complex III and complex IV. Cytochromes receive electron from ubiquinone and pass it to oxygen. Thus, the correct answer is option C.