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

The selected combination is correct because both options 1 and 2 describe scenarios that increase the risk of a semi-trailer truck jack-knifing or skidding on slippery roads.

When the retarder (an auxiliary brake) is fully engaged while navigating a bend, it can cause instability and lead to skidding. Additionally, if the truck is braked harder than the trailer during a bend, it can create a difference in speed and braking force, which can also lead to the truck losing grip and potentially jack-knifing.

These factors are particularly significant under German traffic regulations, which emphasize the importance of safe driving practices, especially in adverse weather conditions. Understanding these connections helps drivers maintain control and enhances road safety for all users.