Final answer:
A horn must be loud enough to be heard from 100 feet away.
Step-by-step explanation:
In order for a horn to be heard from a certain distance, it needs to produce a sound that is loud enough to overcome the distance and any background noise. The sound level decreases as you move farther away from the source. The decibel (dB) is a unit used to measure sound intensity. A general rule of thumb is that the sound level decreases by 6 dB for every doubling of distance.
Based on this information, the horn must be loud enough to be heard from 100 feet away, which is option B. At this distance, the sound level should be high enough to be audible above any background noise and overcome the decrease in intensity due to the distance.
To determine the exact sound level required, additional information, such as the background noise level and the desired signal-to-noise ratio, would be needed.