Final answer:
A motorcycle horn must be audible, under normal conditions, from a distance of approximately 20 meters.
Step-by-step explanation:
The audibility of a sound is determined by its intensity and the sensitivity of the human ear. In the case of a motorcycle horn, it is designed to produce a loud and attention-grabbing sound.
To ensure safety on the road, it is important for other drivers and pedestrians to be able to hear the horn from a reasonable distance. This distance is typically set at around 20 meters, which allows sufficient time for others to react to the sound and avoid potential hazards.