The architect's design for the new museum was a study in contrasts: a facade of austere, unadorned concrete gave way to an interior that was surprisingly opulent and ______.

✓ Correct Answer: Option C

The sentence describes a 'study in contrasts' between an 'austere, unadorned' exterior and a surprisingly 'opulent' interior. 'Sumptuous' (splendid and expensive-looking) is the best synonym for 'opulent' and creates the intended contrast.