Thanks for the question! The problem with (C) is that it’s actually a good contrapositive! Let’s take a look at what it says. It tells us that only oceanographers enjoy the Atlantic in midwinter. Remember that “only” introduces the necessary condition. So we can diagram this
Enjoy Atlantic in midwinter —> Oceanographer
Then, we’re told that Gerald isn’t an oceanographer, so therefore, Gerald doesn't enjoy the Atlantic in midwinter. Well, look at the contrapositive, which is
~Oceanographer —> ~Enjoy Atlantic in midwinter??This is a valid contrapositive! Therefore, it doesn’t match the flawed reasoning, since it isn’t even flawed reasoning in the first place.
Hope this helps! Feel free to ask any other questions that you might have.