EMF becomes equal to the terminal potential difference when the:

Explanation

The EMF (electromotive force) represents the potential difference across the terminals of a battery when no current is flowing. When a circuit is open, no current flows, eliminating any voltage drop across the internal resistance of the battery, making the terminal potential difference equal to the EMF. In contrast, when the circuit is closed or the current is at its maximum, there is a non-zero current, resulting in a voltage drop across the internal resistance. This makes the terminal potential difference less than the EMF, invalidating options A and B. Therefore, option C is the only correct choice.