The correct answer is C. Having a peer in the car.
Teenagers like to rebel and appear cool in front of their peers so they are more likely to do things that are against what their parents are doing or what the law states. If they have a best friend however that does wear their seat belt then they will most likely conform and do the same as they don't want to stand out or seem stupid, also best friends are comfortable enough around each other to not show off. It's different when it's just a friend though, teenagers will try and impress each other over dumb sometimes dangerous things. We've all done it.