Answer :
An appositive is a phrase or word that points to the same thing as another noun in a sentence. It helps in providing additional information about the noun.
Therefore, sentence which uses an appositive phrase that renames the subject to combine the sentences is Washington, D.C., our capital, is fascinating.
Hence, in the above sentence, noun is Washington, D.C. with its addition information of being a capital. The sentence is separated with commas to remove appositive and placed without changing the meaning of the sentence.