Final answer:
The true statement about horns and sirens is that they are crucial for alerting others to your presence, particularly in emergency situations. Horns and sirens are required on vehicles for safety, and using them appropriately is in accordance with the law. The observed frequency of a siren using the Doppler effect matches the siren's actual frequency when the source is directly in front of the observer. Option c) is correct.
Step-by-step explanation:
The correct statement regarding horns and sirens is: c) Horns and sirens are important for alerting others to your presence. Horns and sirens are used on various vehicles, especially emergency vehicles like ambulances and police cars, to alert pedestrians and other drivers of their approach, particularly in emergency situations where they need to move quickly and safely through traffic. Using horns and sirens to express frustration or in non-emergency situations is inappropriate and can be against the law. Horns and sirens are required on vehicles, especially emergency vehicles, for safety reasons.
In terms of the Doppler effect, this is a phenomenon observed when a vehicle with a siren is moving towards or away from an observer. The frequency of the sound changes due to the motion of the sound source. When an ambulance drives by with its siren blaring, the frequency you observe most closely matches the actual frequency of the siren when it is directly in front of you (c. when it is in front of you).