In which scenario is it illegal to use your horn?

Answer: B — In built-up areas during nighttime, unless necessary for safety

Using your horn is regulated to prevent unnecessary noise pollution and maintain a peaceful environment, especially in areas with higher population densities. In built-up areas, specifically during nighttime, the use of the horn is restricted unless it is genuinely necessary for safety reasons. This regulation allows for quieter conditions during hours when most people are asleep, minimizing disturbances while ensuring that drivers can still alert others in critical situations. In rural areas, the rules can be different, and the prohibition on horn use in the presence of a police officer does not universally apply as an outright illegality. Tailgating, although a concern for safety, does not have a specific legal guideline regarding horn usage, making the nuanced regulations around built-up areas at night the better choice.