Which of the following is NOT a requirement for operating a motor vehicle on the highway?

The correct choice identifies that having a working air conditioning system is not a requirement for operating a motor vehicle on the highway. While air conditioning can enhance comfort during travel, it is not essential for the safe operation of the vehicle.

In contrast, the other listed components—speedometer, horn, and muffler—are critical for ensuring the vehicle operates safely and meets legal standards. A working speedometer is necessary for the driver to monitor their speed and adhere to speed limits. A functioning horn is important for safety, as it allows the driver to alert others to their presence or communicate that they are in a potentially hazardous situation. A working muffler is essential as it helps to reduce noise levels and ensures that the vehicle meets exhaust emission regulations, contributing to environmental protection and community comfort.

Overall, while comfort features like air conditioning may be desirable, they do not affect the basic legality and safety of operating a vehicle on the road.