use of the relative pronoun which
When the relative pronoun is the object of a preposition, which is used instead of that, for example,  in which ,  for which ,  about which ,  through which , etc. (please see the third example below): Formal English: This is the man to whom I wanted to speak and whose name I had forgotten.he relative pronoun that can only be used in restrictive clauses. It can also be substituted for who (referring to persons) or which (referring to things) in informal English. Whereas that is often used while speaking, who and which are more common in formal written English.