Pay special attention to the last statement before the question - "He is doing well in school and has a girlfriend who is also a medical student." This sentence is there to tell you that the student in question does not have a disorder; his life is going fine. Answers A, B, D, and E are all disorders, and answer C is "traits", not a disorder. Ergo, the correct answer is C.

"which disorder do they most likely have"

This is a typical type of question in higher level studies, where the scenario given is intentionally inconclusive to measure your ability to determine what path may warrant further investigation.

The correct answer is C. Not because it's a diagnosis, but if there were to be a diagnosis, that would be most likely

Yeah that sounds totally normal. Sounds like he has everything under control. He's actually organized, making lists and shit. He enjoys what he does. He has a girlfriend, so clearly he's not a total hermit. He just chooses not to spend time with collegues. That sounds like a normal well-organized person