Explanation

The hall’s design was likely plain or functional, given the clue minimalist influences.
Choices (A), (B), and (E) are easy eliminations, as they are clearly opposite in meaning. You can eliminate choice (D) if you know that refulgent is the opposite of plain. Unadorned is the best match.

Hence option C is correct!
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Hint: What fits with minimalist

A, D, E - opposite of minimalist
B - out of scope, intricate is complex and it does not fit with minimalist.

C - unadorned perfectly fits and Hence, C is the Answer.