The correct option is B Cytochrome a3
Terminal cytochrome of the electron transport chain (ETC) which donates electrons to oxygen is cytochrome a3 of complex IV.
During the glycolysis and citric acid cycle, NADH+H+ and FADH2 are formed. They get oxidised and release electrons which pass through various complexes present in the inner mitochondrial membrane.
The electrons released by the oxidation of NADH+H+ move to complex I, and then get transferred to ubiquinone located within the inner membrane. Ubiquinone also receives the electrons released by the oxidation of FADH2 through complex II. The electrons from ubiquinone are passed on to cytochrome b and cytochrome c1 of complex III. The electrons are then further transported to cytochrome c. It is a mobile carrier and transfers electrons between complex III and IV. In complex IV, the electrons pass through cytochrome a and then to cytochrome a3. From complex IV, electrons are accepted by oxygen. Oxygen is the final acceptor of the electrons and it gets reduced to water.