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.
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