Before we begin, let's graph ABCD and the line of reflection.
To reflect the square in the given line, the image of each vertex must be plotted on the opposite side of the line of reflection while maintaining the same distance from the line. Notice that B and C are on the line of reflection, and therefore B and B′, and C and C′ are the same points.
Now, we can connect the reflected vertices to have the image of ABCD.