Answer: B. a star at the center of our solar system
Step-by-step explanation:
An appositive is a noun or noun phrase that is placed beside another noun or noun phrase with the intention of renaming it. In order words, the appositive gives further information about the noun or phrase it changes. Appositives usually come between two commas, unless they are restrictive.
In the structure we are analyzing here, the appositive is the sentence "a star at the center of our solar system". Not only it comes between two commas, but it also has the intention of explaining what the sun is by giving further information about it.