Which condition will cause the APU to shut down automatically?

The condition that will cause the APU to shut down automatically is low oil pressure. The APU, or Auxiliary Power Unit, requires a proper oil pressure level to maintain its operation and lubrication. When oil pressure falls below a certain threshold, it indicates that the lubrication of critical components is insufficient. This lack of lubrication can lead to severe mechanical damage, overheating, or system failure. Consequently, to protect the integrity and safety of the APU, an automatic shutdown is initiated to prevent further damage.

While other conditions such as excessive fuel temperature, high altitude, or overheating can impact the APU's performance, they typically don't result in an automatic shutdown like low oil pressure does. Instead, those conditions might trigger warnings or require operator intervention without an immediate, automatic system shutdown. Therefore, low oil pressure is particularly critical because it directly threatens the operational safety of the APU, making it the correct choice for this question.