A fragment is not a main clause because it <em>lacks</em> 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 <em>"There's Jack"</em> and the appositive fragment <em>"The most talented pianist I know."</em> 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.
These two options seem like the best answers due to the passion you get from reading them. I would say the other two don't create enough sense of passion.
