One of the four shapes is not needed to make the shape in the first pic. Which one?
Once again, is it just noticing some properties? Or are there any other logical ways of figuring it out? I tried mentally configurating the given, but really not sure how. I only know that it's not 3), because it has 6 elements, and 15=5+5+6.
Hint:
You've correctly identified one that you do need.
Now, find where that one goes. It can't go just anywhere. (Why not?)
(Actually, the piece can be placed in six different places and orientations, but they're all equivalent.)
From there, you should be able to see easily which two will complete the triangle, and which one can't be used to complete it.