Why Extremely critical and rarely are antonyms?
Firstly to answer your question, I don’t believe Rarely and extremely critical are antonym pairs, rather there is a different relationship between them while theorizing your prediction for the blank.
So if you implement the math strategy/ support or contrast logic then it is obvious that if Author has used Although before saying something, then the exact opposite/ contrast is coming up next. Here he says, although extremely critical, then we can infer that the following sentence or blank is supposed to say “not extremely critical”. We now also have to observe that the word rarely is acting like a replacement for “not” here in the context of “not extremely critical”. Hence with this working, we can look for closest synonym for “extremely critical” viz "D. Contempt: being utterly disrespectful or disapproving"
Please excuse my handwriting, as I figured a visual representation would be helpful to show my thought process. Practicing key words of the sentence like this helps to avoid making mistakes in my personal opinion as GRE is purposefully trying to fool you.
Hope this helps
Jeet.