When is it illegal to pass a school bus?

Answer: B — When the bus is stopped with flashing red lights

Passing a school bus with flashing red lights is illegal because it indicates that children are getting on or off the bus and are in potential danger. Option A, C, and D are incorrect because they do not address the main factor of a school bus with flashing red lights. Option A may seem like a potential answer because if you cannot see the bus, then you may not know that it's stopped. However, if there are no flashing red lights, then it is not illegal to pass. Option C is incorrect because the bus driver does not have the authority to override traffic laws and signal you to pass. Option D is incorrect because flashing yellow lights mean that the bus is preparing to stop, but it is not fully stopped yet, so it is still legal to pass. Therefore, the only correct answer is B, when the bus is stopped with flashing red lights.