Answer:
A: The most talented pianist I know.
Step-by-step explanation:
A fragment is not a main clause because it lacks one of the elements that have to be present: a subject, a verb, or a complete thought. In the case of the option A, we have the main clause "There's Jack" and the appositive fragment "The most talented pianist I know." The correct use of this appositive is to place a comma after the noun Jack, then write the fragment, which has the purpose of describing the noun.