Finally.We have already learned that the relative pronoun that replaces an object can be deleted.This is the case even for elative pronoun that substitute for objects of prepositions. However.remember that deletion is possible only if the preposition has not been fronted along with the relativized object NP:
The place which you spoke about is Denver. The place about which you spoke is Denver.
The place you spoke about is Denver. *The place about you spoke is Denver.