A predicate nominative is also known as predicate noun wich complete the linking verb and renames the subject. It is a complement and they only complete linking verbs.
An example: Students' favorite food must be pizza.
food = subject, must be = verb, pizza = predicate nominative
The correct answer is number 1: An important part of a firefighter’s job is rescuing people
fighterfighter's job = subject, is = verb, rescuing people= predicate nomitative