Polygons should be the correct answer. In my lesson, it asks the same question and the text (reading part) says, "Thus far in our discussion of similarity we have focused primarily on triangles; however, many theorems also deal with similar polygons in general. We shall cover some of them now."
Plus, the lesson name is, "Theorems - Similar Polygons" so I don't understand how the ones who commented below got it wrong...unless they cannot spell.
I assure you, the answer is Polygons.
Have a nice day. :)
-Fishtank
>(^8^)<