1) The musicians is incorrect, because most adjective clauses start with variations of "who" and "where" and "when" and "why" etc...
2) Gave a stellar performance is incorrect for the same reason
3) Who had been practicing is incorrect because it doesn't elaborate on "practicing"
4) Who had been practicing for months is correct because it tells how long they were practicing, and elaborates on the adjective. (practicing)
The sentence that is made up of two simple sentences joined with a comma and a coordinate conjunction is a compound.
I believe the answer is D since it is using exclamation marks , which is used to show astonishment or anger the person is feeling. I hoped this helped.
