The correct answer is B.
In Montreal, my favorite city, many people speak French.
An appositive is bonus information.
An appositive noun or noun phrase follows another noun or noun phrase in apposition to it.
The appositive phrase, usually separated by commas, provides additional information to the main phrase.
In the above sentence, my favorite city, is a appositive noun phrase because it states that Montreal is my favorite city, thus it provides additional information in this sentence.