School buses must stop at railway crossings...?

Answer: B — Under all circumstances

School buses are required to stop at railway crossings regardless of any visible train or flashing lights. This is to ensure the safety of the passengers and to avoid any potential accidents. Option A is incorrect because even if there is no visible train, the bus must still stop. Option C is incorrect because the bus must stop regardless of whether the lights are flashing or not. Option D is incorrect because the bus must stop at all times, not just during school hours.