The answer to the question above is the following one:
a) The concert that I attended was held last August.
A is a restrictive clause because it modifies the noun it precedes. It provides essential information as regards the sentence. If it was dropped, the sentence would not have the same meaning.
b) The audience moved as one, the crowd swaying the music.
B is an absolute phrase. Absolute phrases are phrases that modify a noun in a sentence but they are not connected to the sentence by a conjunction. They are set off with a comma and can be deleted from the sentence without altering its meaning.
c) The concert, which had been rescheduled, was sold out.
C is nonrestrictive clause. It provides information which is considered nonessential. Therefore, it can be erased without altering the meaning of the sentence.
d) The band, a hometown favorite, came back for serveral encores
D is an appositive phrase. An appositive is a noun or a noun phrase that renames another noun right beside it. They are set off with commas.
