Answer:
c. London Underground
Step-by-step explanation:
A "predicate nominative" refers to a noun/pronoun that is used to rename the subject of the sentence. It is always found after the linking verb. Remember that it is part of the "predicate" in the sentence.
The predicate in the sentence above is: "was the London Underground." Therefore, the predicate nominative should be found here. As mentioned above, the predicate nominative should follow after a linking verb. The linking verb above is "was." The only noun following it is the "London Underground." It is also used to rename or identify the subject in the sentence, which is the "first subway system in the world." This makes choice c as the answer.