What must you remember when coupling up a trailer?

The selected combination is correct because both statements address essential safety measures when coupling a trailer. According to German traffic regulations, you are not permitted to connect coupling parts with different colors to ensure compatibility and safety. This rule helps prevent potential accidents caused by improper connections.

Additionally, releasing the parking brake and removing wheel chocks when coupling a trailer is crucial for ensuring smooth operation and preventing unintentional movement of the trailer. This practice aligns with safety standards outlined in German traffic laws, which aim to minimize risks during trailer maneuvering.

The option about connecting the emergency line, while important, is not included in your answer. Connecting the emergency line ensures that the brakes are engaged in case the trailer disconnects while driving, which is a key safety aspect. Thus, the correct approach incorporates all these elements to achieve a safe coupling process.