You must show your driver's license and registration when stopped in a car. Otherwise, you don't have to answer any questions if you are detained or arrested, with one important exception. The police may ask for your name if you have been properly detained, and you can be arrested in some states for refusing to give it. If you reasonably fear that your name is incriminating, you can claim the right to remain silent, which may be a defense in case you are arrested anyway.