Cytat:
While an interrogative clause allows a choice in the placement of the preposition in a wh-element, a nominal relative clause requires the wh-word to be placed first and the preposition to be deferred:
W tym kontekście, czy te zdania są poprawne prócz drugiego?
This is what I have been looking for.
This is for what I have been looking.
This is for which I have been looking.
This is the book (that/which) I have been looking for.
This is the book for which I have been looking.