Which sentence correctly identifies an adverb clause?
the parenthesis are the phrase ()

does the boy (whose kite we borrowed) live in the cottage across the street?

that bracelet, (which is made of sterling silver), has a delicate clasp.

(before we left the camping grounds), we noticed a sign for the scenic trail.

the stuffed plush bear (that randall bought for his baby brother) is soft.

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.