I had to Google for the 'infamous letter C'. In case others were wondering, it seems to stem from folk wisdom still perpetuated that for a blind guess the third answer when given 4 or more options in a multiple choice test is more likely to be correct than the others.

The linked article suggests this does not hold (for the ACT), but doe suggest picking a single letter and sticking with it for all blind guesses would outperform a purely random guessing strategy.

https://blog.prepscholar.com/most-common-answer-on-act