Final answer:
The applied external potential necessary for charging a battery is a positive potential, which is higher than the battery's own electromotive force (emf) to facilitate the nonspontaneous flow of electrons.
Step-by-step explanation:
The applied external potential needed for a charging battery is a positive potential. This potential must be sufficient to move electrons from the positive terminal to the negative terminal, overcoming the internal potential of the battery and driving the nonspontaneous reaction. During the charging process, electrons are forced from the positive terminal to the negative terminal. Since work is being done on the negative charges, energy from an external source is required, and this is provided by the applied positive potential. The electromotive force (emf) is equal to the work done on the charge per unit charge when there's no current flowing, and for charging to occur, the battery's external potential must be higher than the emf of the battery itself. Therefore, the correct answer to the question is B) Positive potential.