To answer this question, the user needs to be familiar with the TV show Friends and its characters.
In the TV show Friends, Joey is one of the main characters and Ross is his friend. Joey believes that Ross's "thing" is not knowing when to shut up, so option D is the correct answer.
Option A (Science) and option B (Academia) are incorrect because although Ross is a scientist and works in academia, these are not considered his defining trait or "thing" according to Joey.
Option C (Being a good father) is also incorrect because while Ross is a father, it is not what Joey considers to be his "thing."
Therefore, the answer is: D. Not knowing when to shut up.