___________ is the terminal cytochrome that donates electrons to oxygen during the electron transport chain.

The correct option is B Cytochrome $a_3$
Terminal cytochrome of the electron transport chain (ETC) which donates electrons to oxygen is cytochrome $a_3$ of complex IV.

During the glycolysis and citric acid cycle, $NADH+H^{+}$ and $FAD{H}_2$ 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 $FAD{H}_2$ 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 $a_3$. From complex IV, electrons are accepted by oxygen. Oxygen is the final acceptor of the electrons and it gets reduced to water.