Answer :
The answer is C. an old upright, because the phrase "an old upright" is second description for "a piano."
Had been stored is the action, the piano the main subject, and in the basement a prepositional phrase.
Had been stored is the action, the piano the main subject, and in the basement a prepositional phrase.
the answer is C. an old upright. is the appositive phrase because it is in ccomas if we remove that part it will still make sence like this. A piano had been stored in the basement.