The correct answer is 'leader'. A predicate nominative, or predicate noun, is the object of a linking verb. When trying to find the predicate nominative, simply use this format: "What did (subject) (linking verb)?"
So, in the sentence "The animal is a cat.", 'animal' is the subject and 'is' is the linking verb.
What is the animal?
The animal is a cat.
Therefore, the predicate nominative is 'cat'.
In tenth grade, Jackie became the leader of the scout troop.
'Jackie' is the subject and 'became' is the linking verb.
What did Jackie become?
Jackie became the leader. (We can exclude 'of the scout troop' because it is a prepositional phrase.)
Therefore, the predicate nominative is 'leader'.
Hope this helps!