What indicates inadequate engine performance while in flight?

Inadequate engine performance while in flight is indicated by engine RPM falling below normal operating limits. This is crucial because engine RPM is a direct measure of the engine's power output and efficiency. When the RPM is below normal, it suggests that the engine isn't producing the required thrust for flight, which can lead to a loss of altitude, reduced climb rate, and potential control issues.

While the other options can affect the aircraft's operation, they do not directly indicate engine performance issues. A decrease in cabin pressure primarily suggests a problem with cabin pressurization rather than engine efficiency. Increased fuel consumption might indicate various factors impacting performance, but it does not directly reference the engine's capability in terms of power output. Fluctuations in airspeed could result from multiple factors, including atmospheric conditions or pilot inputs, rather than a specific engine performance issue. Therefore, the clear indicator of inadequate engine performance while in flight is indeed when the engine RPM falls below the normal operating limits.