In the second part of the question, I actually spotted the word “impossibly rude” instead of “rude” and ended up doing the opposite thus choosing coarseness as my answer. Can someone help me know why we chose rude and not impossibly rude. I am a bit confused here.
In this question it does not matter if you consider “impossibly rude” or “rude”, just check if modifiers are contrasting or supporting.
Sadly in this question, you misunderstood “impossibly”. Its not contrasting “rude” part.
Here’s its definition from oxford
impossibly /ɪmˈpɒsɪbli/ adverb
-
- so as to be impossible.
“every task seemed impossibly difficult”
As per your understanding, this above sentence should mean easy but its not, its just supporting difficulty part with added emphasis. This 2.2 definition makes the same point clear
-
- possessing the specified quality to an unbelievably high degree.
“her impossibly blonde hair”
Nevertheless, whenever you are confused, look at the extra part which they have given, here it is
at least harmlessly bizzare
Now this should mean the same thing as impossibly rude to some degree, coarseness doesn’t fit in because its not contrast to harmlessly bizzare, as harmlessly bizzare is still bizzare to some extent
I hope its clear now:)
Yes it’s all clear now… thank you so much! 
You are welcome