What is the function of the missing word in the following sentence?

to is this letter being sent?

subject
predicate noun
direct object
object of the preposition

btw it's not subject.

M read the sentence.what best describes the underlined part of the sentence,and why? alma plans to visit london and paris, where her parentswent on their first trip together as a married couple.it is a restrictive relative clause because it begins with arelative pronoun, has both a subject and a verb, and is notnecessary to the meaning of the sentence.it is a nonrestrictive relative clause because it begins witha relative pronoun, has both a subject and a verb, and isnecessary to the meaning of the sentence.it is a restrictive relative clause because it begins with arelative pronoun, has both a subject and a verb, and isnecessary to the meaning of the sentence.it is a nonrestrictive relative clause because it begins witha relative pronoun, has both a subject and a verb, and isnot necessary to the meaning of the sentence.