You must sign the previous driver's inspection report only when:

The correct answer to this question is based on the requirement for a driver to acknowledge that the vehicle is in safe condition before starting their route. When you sign the previous driver's inspection report, it signifies that you have reviewed the report and that any defects that were noted have been addressed. By signing, you are confirming that the defects reported have been repaired, ensuring that the vehicle operates safely for the passengers.

This practice is crucial because it helps maintain a chain of accountability regarding vehicle maintenance and safety. It is essential to ensure that a bus is fit for transporting passengers, and acknowledging that all reported issues have been corrected is a fundamental part of that process.

The other options do not hold when it comes to signing the inspection report. For instance, signing when repairs have not been made would not accurately reflect the vehicle's status. Similarly, the presence of the previous driver or the fact that all passengers have boarded does not relate to the condition of the vehicle or the inspection report's integrity. Thus, the requirement to sign is centered firmly on the condition of the vehicle and the resolution of reported defects.