The bolded word here is the BESTSELLER. This kind of complement is a direct object complement. The adjective overnight denotes to the object bestseller which is an adjective. Another adjective complements another adjective, thus, the term direct object.
The group of words that will describe the subject.
A predicate noun is a word which renames the noun that has been mentioned in the sentence. In this case Franklin Roosevelt is the noun that has been renamed as "President", which makes it the answer to the question.
Another example in : My brother's favourite food is pizza.
<span>The predicate nominative in the sentence is Pizza. because that is the word that is replacing the earlier mentioned noun, which was "food". </span>
The answer is C. It makes the poem sound more like a conversation between the poet and reader.