To drive on a **poorly lit interurban road** at a speed of **40 km/h**, it is necessary to have the **low beam headlights** on. These lights are mandatory on roads both inside and outside towns and cities during the night. Additionally, they must be used when driving at less than 40 km/h, which is the case here.

The **high beam headlights** are not mandatory at this speed, as their use is optional when driving at less than 40 km/h. On the other hand, **position lights only** are not sufficient to ensure adequate visibility on a poorly lit road. Therefore, the correct option is to use the **low beam headlights at least** to ensure safe driving in these conditions.