The correct answer is D) which had undergone a complete make-over.
An adjective clause is, basically, an expanded adjective. It provides a description of the subject or object of the main clause. For example, this adjective clause could easily be replaced with a simple adjective. It is a clause, and not a phrase, because it has a predicate ("had undergone"). It is a dependent clause because it can't stand on its own, unlike the main clause.