None. Parents should *never* buy a car for their children. The teen needs the experience of saving for a beater car and paying their own insurance, or they will never learn the value of a dollar or a car.
My daughter worked all summer to save $1000 for a car. I went around with her and we eventually found a Corolla she could afford. It was a few months younger than she was. We had already worked out the details: she would buy the car outright, she would pay the insurance, and I would help her do the repairs it needed and the maintenance. Today she knows more about cars, including buying and caring for them, than her husband does. A few months ago, nearly 20 years later, she helped me change out the drive axles on her CR-V.
You may not want to go to that extent but you would not be doing her a favor leaving her out of the long, hard path to car ownership. She has to learn sometime - why not now?