A "predicate nominative" refers to a noun/pronoun that is used to rename the subject of the sentence. It is always found <em>after the linking verb. </em>Remember that it is part of the "predicate" in the sentence.
The predicate in the sentence above is:<em> "was the London Underground." </em>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<em> "was."</em> The only noun following it is the<em> "London Underground."</em> 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.
At the end of Sophocles' Oedipus Rex, the protagonist Oedipus has gouged out his own eyes and is sent into exile. His wife (and -- spoiler alert -- his mother) is dead, having hung herself.
