Which kind of complement are the underlined words?
isn't this movie the black stallion?
a. indirect object
b. predicate nominative
c. predicate adjective
d. direct object

b. predicate nominative

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". 

Explanation:

personal pronouns ( We, you)

possessive pronouns ( yours, our, my, their)