Wich set of shapes contains members that are always similar to one another

C) Equilateral Triangles

Any two triangles are similar if they have the same corresponding angles. In the case of two equilateral triangles, they both have 3 sets of 60 degree angles. So the angles pair up and we can prove them to be similar using the Angle Angle (AA) Similarity theorem. 

This trick only works with triangles. It partially works with rectangles; however, we would need to be sure that the corresponding sides were proportional as well. There isn't a 100% guarantee that any two rectangles are similar. 

Could you make it more clear?

Ithink you’re answering is e