Your tone
Let's look at the other choices first. Your credibility is your ability to be believed as trustworthy in regards to the subject. Whether you're talking to a group of teenagers or professional adults, your credibility doesn't change. Your clarity also should not change based on the audience because you want to be clear in your message at all times. Also, your purpose should be the same no matter the audience. The only thing that would change is your tone. The way you approach a crowd of teenagers would vary greatly from the way you'd approach a room of professionals.
Explanation:
Receptionist- Hello! this is your local doctors office! Chelsea speaking how may I help you?
Me' - Hi, I need to make an appointment with Dr. Thomas.
Receptionist- Ok, What's the appointment for?
Me' - A follow up on my diagnosis with COPD
Receptionist- Ok, when can you come in?
Me- Nex Friday.
Receptionist- Okay. that will work. He may want to do an X-ray on your lungs. When you arrive bring your insurance and be 15 minutes early to fill out paper work. Any questions?
Me. No. thank you!