Exemplar basically means a typical example, or model, of a concept or role. However the meaning can be extended to mean he best example of a concept or role. For instance, someone might say that you are an exemplar student, which can mean that you are a great example, or model, of a student.
The correct answer is becoming an astronomy researcher
For starters, it is the only appositive phrase. It is also non-essential because the sentence conveys full meaning even if it was removed.
