The central idea is the central, unifying element of the story, which ties together all of the other elements of fiction used by the author to tell the story.
B. Is probably the answer
example, Mary, a farmer child, walked up to me.
The answer is A. Mercedes threw Andrea, Mercedes' best friend, a surprise party last night. This choice uses an appositive ("Andrea, Mercedes' best friend,") to combine the two sentences. An appositive is a phrase separated from the rest of the sentence by commas that describes something (in this case Andrea).
Choices B and C are incorrect because the uses of the words "but" and "when" to combine the sentences do not make logical sense. Choice D is incorrect because it uses a comma to combine the sentences without a conjunction, creating a run-on sentence.
Hope this helps!