For an underage driver, a blood alcohol concentration (bac) of gives california authorities the legal right to arrest the individual.

0.05%

Explanation:

Californian authorities can arrest a  driver under the age of 21 for driving under the influence of Alcohol if his/her BAC is 0.05% or higher. The illegal BAC percentage for a driver over the age of 21 is 0.08%.

But even less blood alcohol concentration is considered illegal. For example, for drivers under the age of 21, a BAC of 0.01% is enough to revoke their right to drive for a period of 1 year.

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