Which of the following is a common indication of an APU fire?

An illuminated fire light serves as a direct and standard indication of an Auxiliary Power Unit (APU) fire. This light is part of the aircraft's warning systems designed to alert the crew to a fire condition. When the APU experiences a fire, the corresponding sensors activate and trigger the fire light, which is specifically engineered to ensure quick recognition of a potentially dangerous situation. Recognizing this indicator is crucial for immediate response actions to mitigate risks and ensure passenger safety.

The other options, while they may suggest issues or malfunctions, do not directly pertain to the specific and immediate indicator of an APU fire. A decrease in power may indicate operational performance issues but does not specifically signify a fire. A sudden drop in cabin pressure could relate to various problems but is not a typical sign of an APU fire. Similarly, abnormal engine noise could indicate a range of issues but is not a direct indicator of an APU fire, making the illuminated fire light the most reliable signal in this scenario.