To be able to answer your question, let me first refine the grammar and syntax of that sentence you presented. It should be reworded as follows to be analyzed properly: “Are you the member of the Church of Christ who (is, are) a student of Bulacan State University?”
With the question reworded that way, it will achieve subject-verb agreement by using the singular “is” instead of “are”: “Are you the member of the Church of Christ who is a student of Bulacan State University?”
That sentence has for its subject complement the entire noun phrase “the member of the Church of Christ who is a student of Bulacan State University.” In English grammar, a noun phrase is categorized as a nominal group, which by definition consists of a head noun and all the other words that modify or characterize that noun. The words that precede the head noun are called its premodifiers, and the items that come after it are its qualifiers. In the noun phrase “the member of the Church of Christ who is a student of Bulacan State University,” the head noun is logically the word “member,” the word “the” that precedes it is its premodifier, and the words “of the Church of Christ who is a student of Bulacan State University” that come after it are its qualifiers.
Grammatically, it is the head noun that determines whether the noun phrase is singular or plural. Indeed, in a noun phrase, the form of the operative verb is always determined by the number of the head noun—the verb takes the singular form when the head noun is singular and takes the plural when the head noun is plural. Any other noun or pronoun found in the premodifier or in the qualifier of the head noun doesn’t determine or affect its being singular or plural.
In the noun phrase “the member of the Church of Christ who is a student of Bulacan State University,” the head noun “member” is singular, so it requires the singular form “is” of the verb “be” rather than its plural form “are.” (The pronoun “you,” which requires the plural form “are” of the verb, shouldn’t be mistaken for the head noun here because it isn’t really part of that noun phrase.) This, in sum, is why the singular “is” is the correct form of the operative verb in the question “Are you a member of Church of Christ who is a student of Bulacan State University?”
