The word President is the predicate nominative here, also known as predicate noun. 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. The predicate nominative in the sentence is Pizza. because that is the word that is replacing the earlier mentioned noun, which was "food".
