Final answer:
Ubiquinone has a lower redox potential than cytochrome c, and therefore electrons flow from ubiquinone to cytochrome c in the electron transport chain.
Step-by-step explanation:
The roles of ubiquinone and cytochrome c differ from the other components of the electron transport chain in terms of their redox potentials and the direction of electron flow. Ubiquinone has a redox potential of +30 mV, while cytochrome c has a redox potential of +230 mV. Electrons flow from ubiquinone to cytochrome c, making option A - Ubiquinone to cytochrome c - the correct answer.