Which word in the sentence is a predicate nominative? franklin
d. roosevelt was the only president of the united states to serve three terms.
a. president
b. only
c. terms
d. three

The answer is a. President.

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". 

Syllable added to a root word to change its meaning : affixan independent word--example run,walk,book : free morphemeaffix that gives grammatical meaning such as tense or number : inflectiona word part that must attach to a word in order to be usable : bound morphemedegree of comparison formed by adding -er or more to an adjective : comparativedegree of comparison formed by adding -est or most to an adjective : superlativeaffix added to the beginning of a word : prefixaffix added to the end of a word : suffixroot word of lovable : lovetwo roots used together to form one word : combining form

Not sure brra