Ok, so you're being show a set of triangles and the only information you have about them are the lengths of their sides. This means that there is a relation between the sides of one of this triangles with the sides of the XYZ.
Now, if you look at triangle C you can see that their sides have exactly the double length of the sides of triangle XYZ. This means that the angles of triangle C are the same that those of triangle XYZ so they are similar.