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.
<span>D. Arsat’s brother was holding their pursuers off with a gun.
According to the excerpt the brother wanted Arsat to continue on so he could keep the people away. He says that they do not have a gun and they'd be intimidated. He says that once he has "fired all the shots" he will catch up since he's a strong runner.
</span>
