To solve this problem, we need to determine the relationship between the current $I$ and the number of cells $n. \\$Given:$\\$• Each cell has an internal resistance $r. \\$• The cells are connected in series.$\\$When the terminals of the battery are short-circuited, the total internal resistance $R_{total}$ is:$\\ R_{total} = n \cdot r \\$The total electromotive force (emf) of the battery is $E_{total} = n \cdot E,$ where $E$ is the emf of each cell.$\\$Using Ohm's Law, the current $I$ is given by:$\\ I = \frac{E_{total}}{R_{total}} = \frac{n \cdot E}{n \cdot r} = \frac{E}{r} \\$This shows that the current $I$ is independent of $n. \\$Therefore, the graph of $I$ versus $n$ is a horizontal line, indicating that $I$ remains constant as $n$ changes.$\\$The correct graph is Option 3, which shows a horizontal line.