At what speed is a motorbike not allowed to travel on this road?

In the photo you are driving on the A4 motorway - you can see a board with the number A4 and motorcyclists in the left lane. You are not allowed to ride on this road at 80 km/h if you are carrying a six-year-old child - answer C.

The Road Traffic Act states that the speed limit for a motorcycle on a motorway is 140 km/h, but for a motorcycle carrying a child up to 7 years old it is 40 km/h, no matter what type of road it is. With a six-year-old, 80 km/h is therefore exceeding the limit by 40 km/h, although without the child it would be legal. Answer B is wrong, because 140 km/h is exactly the motorway limit for a motorcycle. Answer A is also wrong: the Act does not prohibit riding at 20 km/h "regardless of the conditions" - in a traffic jam or on ice such slow riding may be necessary, and the Act only requires you to drive at a speed that does not obstruct others, so what is prohibited is riding so slowly without good reason that it obstructs traffic. In practice: you do not go onto a motorway with a small child, because a motorcycle travelling at 40 km/h among cars at 140 km/h is deadly dangerous - you choose local roads to get to your destination.

Legal basis: Art. 20 para. 3 point 1 letter a and para. 6 point 3 and Art. 19 para. 2 point 1 of the Road Traffic Act (Prawo o ruchu drogowym)