The correct answer is predicate nominative.
In grammar, a predicate nominative is the first noun (or a noun phrase, in this case) that comes after a linking verb. If you take a look at the example above, you will see that the linking verb is IS and the first noun phrase that follows it is one of Mark Twain's most famous books, which means that the phrase is a predicate nominative.
The subject of the sentence would be The adventures of Tom Sawyer rather than the second phrase.
