The correct option is C a unique
Some words do not have a comparative or superlative form, they cannot show a greater or lesser amount. 'Unique' is one such word, which means 'one of a kind', and does not have a superlative degree as shown in the sentence. Hence option A is incorrect. Option B is incorrect because 'the' is used for superlatives, and the following adjective isn't a superlative. Option C is correct because the article is indefinite which is used for non-specific nouns. The adjective 'unique' following it modifies a noun that is non-specific (style). Option D is incorrect because 'unique' is an absolute modifier, i.e. an adjective which cannot be modified by 'very' or any similar word because it's meaning is so specific that it does not need to be modified.