The answer C correctly uses possessive form and is the right answer. It's not A because there is only one George so the apostrophe is in the wrong place. In B, there's not even an apostrophe and it's not clear as to whether there is only one cousin or two or more cousins. In answer D, here should not be an apostrophe for develops.
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.
