The correct option is D good
A subject complement is an adjective, a noun or any phrase that follows a
linking verb and gives more information about the subject.
In the given sentence, "good" is an adjective that gives more information about the subject, i.e., "The music". It follows the linking verb, "sounds". Hence it is the subject complement and option D is correct. "The music" is the subject itself, "in the newly opened diner" is a prepositional phrase that doesn't follow the linking verb and "sounds" is a verb. Hence options A, B and C are incorrect.