What applies to the operation of vehicles with regard to the height of the vehicle, including the height of the load?

The combination of choices you selected is correct because they cover essential aspects of compliance with vehicle height regulations in Germany.

1. Checking the vehicle height in Part I of the registration certificate is important as it officially states the vehicle's dimensions, ensuring you know what specifications you are working with legally.

2. Determining the actual height of loads that extend above the vehicle is crucial because any load exceeding the defined height may lead to legal consequences, traffic hazards, or accidents.

While choice three, about complying with the maximum height of 4.5 meters, is also relevant, the first two points ensure you are aware of your specific vehicle's capacity and how any loads you carry can affect your overall compliance with traffic laws. Therefore, it is important to select all pertinent options to ensure complete adherence to regulations.