A line of reflection is a line that make the figure in two equal identical parts.
A line of reflection divide a figure in two equal and identical parts so that those divided parts looks like a mirror image of each other.
In the given figure, we have r, s and t lines.
It's clear from the figures given on the diagram, that only line r is cutting given figure.
Now, we need to see, if that line r cutting the figures make the mirror images of cutting image.
It can be seen clearly that top part of the images are the mirror images of bottom parts.
Therefore, only line r is the line of reflection for the pair of figures.