What is prohibited when crossing railway tracks?

Answer: B — Changing gear

Crossing at an angle is prohibited when crossing railway tracks because it can increase the risk of a collision with oncoming trains. Speeding is also prohibited, but it is not the most immediate safety concern when crossing railway tracks. Using horns is not allowed near railway tracks, but it is not strictly related to the actual crossing itself. Changing gears, however, is the most important prohibition when crossing railway tracks as it is essential to maintain a steady speed and avoid getting stuck on the tracks. Therefore, option B is the correct answer.