Answer:
Ubiquinone.
Step-by-step explanation:
Electron transport chain is the important cycle in which electrons are transferred are from one complexes to another complexes and couples it with proton transfer. This is important for the formation of ATP molecule.
Ubiquinone acts as a mobile carrier in the electron transport chain. The ubiquinone accepts electron from the NADH dehydrogenase. The accepted electron is then transferred to the complex III ( cytochrome C reductase). Both oxidation and reduction occur in this step.
Thus, the correct answer is option (a).