Rhombus
It's quite obvious what the answer to level 18 is. Since that answer is being rejected, I can only say that this was a poorly made game.
To be more precise, the level design was poor.
Rhombus, Kite, Diamond, Square. I should not have to try further on this level to guess the right word for Rhombus that you want used.
I give you a two for the previous levels being decent and the overall design and presentation being smooth.