The electromotive force (emf) of a cell is defined as the maximum potential difference that the cell can provide. This maximum potential difference is measured across its terminals when no current is

The electromotive force (emf) of a cell is defined as the maximum potential difference that the cell can provide. This maximum potential difference is measured across its terminals when no current is flowing through the external circuit. This condition occurs when the circuit is open.

In an open circuit, there is no voltage drop due to the internal resistance of the cell, so the terminal potential difference is equal to the emf. When the circuit is closed, current flows, and the terminal potential difference is less than the emf due to the voltage drop across the internal resistance ($V = \text{emf} - Ir$).

The final answer is $\text{A) open circuit}$.