He has been groaning for more then a year.
"Did you groan?" ask Dorothy
"I did. I've been groaning for more then a year, and no one has ever heard me before or come to help me."
Answer:
I think it's going to be C
An adverb clause, or a subordinate clause, is the one which cannot exist on its own, it has to be a part of the main clause, or else it sounds incomplete. The only such example here is B, because if you look at that clause on its own, it needs to be completed.
A is just a weird construction meaning nothing. C is a fragment of a sentence. And D is an infinitive phrase.
