D) Stimulus is the answer, because the stimulus is what gives us the quick reactions to things we see, hear, and smell. Like if someone were to punch you, your instinct would be to dodge it or put your hands up. If you heard a loud sound you would like flinch or cover your ears from instinct. Also, if you were to smell something your natural instinct is to cover your nose, gag, or say something about it.
Hope This Helps :)
