The correct answer is option letter C subject complement. In the sentence “And, once upon a time, such words were magic archways into a world of fantasy…”, the subject is “such words” and the verb that follows this subject is a linking verb (verb "to be" in the simple past tense). Therefore, a subject complement is needed. In this case the complete subject complement is “magic archways”, in which the countable noun “archways” is modified by the adjective “magic”. The word in bold “archway” is part of the subject complement.
