The correct answer is - (Before we left the camping grounds), we noticed a sign for the scenic trail.
An adverb clause functions the same way a simple adverb would - it describes the time, place, manner, etc. of the verb, of the action that is taking place. The only option that does this is C - before we left the camping grounds is an adverb clause which tells us when the action of noticing a sign happened.
Options A and B are adjective clauses; option D contains a noun clause.
