In an experiment, a student in the laboratory measures the terminal potential difference (V) of a cell (emf E and internal resistance r) as a function of current I flowing through it and plot a graph taking V on y-axis and I on x-axis. What is the slope of the graph between V and I?

$r^2$ $-r^2$ $r$ $-r$

$-r$

The correct answer is Option (4) → $-r$