What is the first sign of a train approaching at a level crossing?

Answer: A — solid amber light

When approaching a level crossing, a solid amber light is the first indication that a train is approaching. A flashing red light would signal the train is already at the crossing, and a green light signals that it is safe to cross the tracks. A bell sound is not always a reliable indication of a train's approach at a level crossing, as it may also be used to signal other vehicles or pedestrians. Therefore, A is the best answer as it is the most accurate and reliable indication of a train's approach.