What alterations can result in the permit for use (Betriebserlaubnis) being cancelled?

In Germany, the Betriebserlaubnis (permit for use) can be cancelled if changes are made to a vehicle that affect its original specifications or safety. The selected options are correct because:

1. Fitting a sidecar changes the vehicle's configuration and can impact stability and handling, influencing its compliance with safety regulations.
2. Fitting tyres of a different size can affect the vehicle's handling characteristics, speedometer accuracy, and overall safety, which are crucial for maintaining the vehicle's approved operating conditions.

It is essential to adhere to these regulations to ensure the vehicle remains safe for public use and compliant with legal standards. Not addressing these alterations can lead to serious safety issues and legal consequences. The third option, fitting a rear fog lamp, does not affect the Betriebserlaubnis unless it violates specific installation guidelines, which is why it is not included in the correct answer combination.