Which of the following factors determines to the greatest extent the length of an agricultural tractor's braking distance during emergency braking?

The stopping distance depends directly on the grip of the tyres on the road surface. This grip is primarily determined by the type and condition of the road surface, as well as the prevailing weather conditions. The snow visible in the picture by the roadside suggests a low temperature, which increases the likelihood of a slippery road surface and a significant increase in the stopping distance.