The electromotive force of a battery is the voltage between its terminals when

Explanation

The electromotive force (emf) of a battery is defined as the potential difference across its terminals when no current is flowing through the circuit, which is when the circuit is open. This is because the emf represents the maximum potential difference provided by the battery, without any voltage drop due to the internal resistance. When the circuit is closed, current flows, and the measured voltage is reduced by the internal resistance. Therefore, option B is correct, while the other options describe conditions where the voltage is affected by the current and internal resistance.