Final answer:
The word in question is 'Crass'; it describes crude behavior and when modified by adding 'un-' hypothetically indicates the opposite, which is sophistication.
Step-by-step explanation:
The 5-letter word that describes crude behavior in high society when preceded by 'un-' but means showing sophistication without 'un-' is Crass. When 'un-' is added to make 'uncrass', which is not an actual word, we can infer it would mean the opposite of 'crass', suggesting sophistication or refinement. The correct answer to the question is c) Crass.