Which of the following can cause an APU to shut down unexpectedly?

A temperature regulation failure can indeed cause an APU to shut down unexpectedly. The APU relies on maintaining specific temperature thresholds to operate efficiently and safely. If the system fails to regulate temperature, it may either overheat or operate outside of its designated temperature range, prompting an automatic shutdown to prevent damage to the unit. This feature is critical for ensuring the reliability and longevity of the APU, as uncontrolled temperatures can lead to significant mechanical failures or safety hazards.

While other factors like excess engine temperature or low passenger capacity can influence APU operation or usage, they are less direct causes of an unexpected shutdown of the APU itself. Lack of flight preparation typically relates more to procedural issues rather than a direct mechanical failure. Temperature regulation is a core operational aspect that directly affects APU functionality, making this the most pertinent choice.