The definition of an appositive is a word or word group that defines or further identifies the noun or noun phrase preceding it. In the given question, 'the Polish virtuoso pianist' is the appositive that defines the noun 'Chopin'.When the noun preceding the appositive provides sufficient identification on its own, one must use commas around the appositive. In accordance with the above stated reason, option A is the correct answer.
Options B, C and D are incorrect because they are not in accordance with the above mentioned condition.