The appositive phrase defines the term asterism. It tells the reader that an asterism is an "informal constellation name".
Appositive phrases are often surrounded by commas. They follow a noun and rename or describe it. Another example is "My dog, a yellow lab, loves to play ball." In this sentence the appositive phrase is "a yellow lab". It describes the noun - dog using a noun - lab.
c. the governor spoke at length about the problems facing the nation's poor. this statement makes it clear that the person speaking had listened to what the governor said and wasn't speaking subjectively. they stated a fact and not an opinion, making this the correct answer.
I think that would be C because you still want to have a good essay! Hope i helped! :)