Which of the words below is an adjective that was made by adding adjective-forming suffix to word root aud which means sound?

You're looking for an adjective formed by adding a suffix to the root "aud" meaning "sound". Here are some possibilities and why they fit or don't fit the criteria:

* Audacious: This word comes from the root "aud" but is not an adjective, it's an adverb meaning "daring" or "bold".

* Auditory: This is the correct answer! It uses the root "aud" and adds the adjective-forming suffix "-ory" meaning "relating to".

* Audible: This word also uses the root "aud" and adds the adjective-forming suffix "-ible" meaning "capable of being".

So, the answer is auditory.