When is there a particularly high risk of a semi-trailer truck with compressed-air brakes jack-knifing or skidding on a slippery road?

You answered correctly! Both situations—if the retarder is fully engaged when driving around a bend, and if the truck is braked harder than the trailer in a bend—create a high risk of jack-knifing or skidding for a semi-trailer truck on a slippery road.

This is because, in both cases, the braking force acts mainly on the tractor unit. If the tractor slows down much more than the trailer, there is a risk that the trailer could push sideways, causing the truck to fold or jack-knife. This risk is especially high on slippery surfaces where traction is lower.

The third answer, about the load impinging on the drive shaft, refers more to weight distribution and traction, but does not create the same direct risk of jack-knifing when braking in a bend.

To solve similar questions, focus on how braking forces are distributed between the truck and trailer, and remember that uneven braking or using certain braking aids like the retarder in bends can be dangerous on slippery roads.