Question

Identify the predicate nominative(s) in the sentence. In tenth grade, Jackie became the leader of the scout troop.

Answer 1

The answer to your question would be "Leader"

Answer 2

I agree with the person above.
A predicate nominative is the first noun after a linking verb such as to be, or in this case became. So, having this in mind, the predicate nominative in this sentence is the word leader.