The sentence that employs the appositive correctly would be:
B). For the middle of the afternoon, the pool was unusually quiet, a quiet oasis of beauty.
What is an appositive?
An appositive is described as the word or phrases(noun) that are placed next to one another where one defines or elaborates the other.
In the second sentence, the phrases "the pool...quiet" and "a quiet...beauty' are the nouns that provide detail about the noun in distinct aspects.
The other options involve a modifier but not an appositive as they include the singular description.
Thus, option B is the correct answer.
Learn more about "Appositive" here:
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