This is probably one of the best questions in the quiz. Have a good look at it. When the answer finally clicks, you will appreciate why I don't want the answers plastered all over the internet. This quiz has been too well set to be destroyed by lazy thinkers.
I have been thinking about 'layer' etc., but without success so far. And I didn't look earlier because I was - and am - still trying. As for 'lazy thinker', I refuse to be goaded but I refute that accusation completely.
Now that you have found it, do you agree that it is a very well set quiz and shouldn't be destroyed by people posting answers. If they can't do it, so be it, leave it for those who can.