Which type of lights must be functional on a commercial vehicle?

The answer focuses on the vital safety and functionality requirements for a commercial vehicle. Headlights are essential for visibility during nighttime or low-light conditions, allowing the driver to see the road ahead and be seen by others. Brake lights are crucial for signaling to other drivers when the vehicle is slowing down or stopping, thereby preventing rear-end collisions. Turn signals are mandated for indicating the driver's intentions to change lanes or make turns, which enhances safety by providing clear communication with other roadway users. Clearance lights are important for making the vehicle visible from a distance, particularly on wider or longer commercial vehicles, ensuring they are seen in various conditions and reducing the risk of accidents.

While the other sets of lights mentioned in the other choices do serve important roles, they do not encompass the core requirements necessary for safe operation of commercial vehicles as outlined by regulations.