The answer is D: Galileo.
Appositive is a noun or noun phrase that renames a noun beside it, usually a noun that comes before it in a sentence. Appositives can be essential (it contains information necessary for the meaning of the sentence.) or nonessential (the information is not needed for the complete meaning of the sentence.) Galileo is necessary because it helps to know what "famed astronomer" the writer is talking about.
