I hit c) for the category theory course in my masters. I managed the first half, more or less, but it felt like it was ramping up exponentially; there were too many new layers of concepts all of which were defined in terms of the previous one, and every new layer meant a percentage slowdown in my ability to work with that concept.

During undergrad I'd been at about the 30th percentile, but only the best half of undergrads go on to do a masters (at least at that particular institution). In retrospect it shouldn't have been a surprise that I was towards the bottom of the class, but it was.