Each part and area of the ear have specific functions in the hearing process. in which of the following parts of the ear are sound waves changed into a signal the brain can understand?

There are no "following" parts of the ear listed.

Sound waves are changed into a signal the brain can understand, consisting of nerve impulses, in the "hair cells" in the lining of the cochlea ... that spiral thing deep in the inner ear.

(In fact, that's as deep as you can get in the inner ear, because it's literally the end of the ear.  After those hair cells inside the spiral cochlea, there's no more ear.  The sound has now changed to electrical signals that go from there to the brain, in the form of electrical currents flowing in nerves.)

answer: the answer is a