Hey there!
BAC, or blood alcohol concentration, basically measures the amount of alcohol present in someone's blood. Previously, for underage drivers, the maximum amount of alcohol they could have in their body was 0.02%. However, in many states in the U.S., there have been laws passed making it illegal for an underaged driver to drive with a BAC anything above 0.00%.
So, depending on the context of your question, the blank might be "legal" or "illegal". But, since the zero tolerance laws haven't been passed everywhere in the U.S. and 0.0001% is such a minuscule amount, I would say the answer is "legal".
Hope this helped you out! :-)