Which type of horn must be working during a pre-trip inspection?

Answer: C — At least one of the horns

During a pre-trip inspection, it is essential that at least one of the horns is operational. The primary purpose of the horn is to alert other drivers and pedestrians, enhancing safety on the road. In most vehicles, including those requiring a Class A CDL, a functioning horn is a critical safety requirement. Having at least one horn working ensures that the driver can communicate effectively with others, which is crucial in preventing accidents. Whether it is the electric horn, the air horn, or a combination of the two, the key aspect is that at least one is functional to comply with safety regulations and maintain operational readiness of the vehicle. Focusing on the other answer choices, suggesting only the electric horn to be functional downplays the importance of having an alternative horn option available, particularly in vehicles equipped with two types of horns for versatility in communication. Similarly, requiring both horns to be operational at all times could pose an unnecessary barrier to passing the inspection, as long as one is working effectively, the safety requirement is met.