Alma plans to visit London and Paris, where her parents went on their first trip together as a married couple. What best describes the underlined part of the sentence, and why?

It is a restrictive relative clause because it begins with a relative pronoun, has both a subject and a verb, and is not necessary to the meaning of the sentence.
It is a nonrestrictive relative clause because it begins with a relative pronoun, has both a subject and a verb, and is necessary to the meaning of the sentence.
It is a restrictive relative clause because it begins with a relative pronoun, has both a subject and a verb, and is necessary to the meaning of the sentence.
It is a nonrestrictive relative clause because it begins with a relative pronoun, has both a subject and a verb, and is not necessary to the meaning of the sentence.