What is required on the side of every truck-tractor?

For truck-tractors, there are specific requirements in terms of lighting and reflectors to ensure visibility and alert other drivers on the road. Among the given options, the one that correctly identifies what is required on the side of every truck-tractor is having one amber side marker lamp.

The function of the amber side marker lamp is to indicate the width and presence of the truck-tractor, particularly during low visibility conditions. This contributes to overall road safety by making the vehicle more visible to other drivers, especially at night or in inclement weather.

While other vehicles may have varying requirements regarding lighting and reflective devices, the specific need for an amber side marker lamp is a standard regulation for truck-tractors. This lamp helps prevent potential accidents by ensuring that the truck-tractor can be seen by other motorists, thus facilitating safer navigation around larger vehicles.