Which word in the sentence is the predicate nominative?

Sugar is the main ingredient in taffy.

a. sugar

b. ingredient

c. main

d. taffy

Predicate nominative is also popularly known as the predicate noun. It is a noun, a pronoun or any nominal that follows the linking verb and most of the time the form of the verb be. It links or completes the linking and change the name of the subject. It complements or completes the verb in the sentence. However, predicate nominative completes only linking verbs. Predicate nominative can be compound while some do not have one. Therefore in the sentence "Sugar is the main ingredient in taffy", the predicate nominative is "ingredient".