Which of the following indicates that the hydraulic filter is functioning properly?

The indicator that signifies the hydraulic filter is functioning properly is when the filter bypassed light is off. This light serves as a warning that the filter may be clogged and that fluid is bypassing it, which could potentially lead to unfiltered fluid circulating in the hydraulic system. When the light is off, it means that the filter is allowing fluid to flow through it normally without any restriction, indicating that it is not clogged and is operating correctly.

In contrast, the other options do not specifically indicate the functionality of the hydraulic filter. While pressure readings and flap movement could provide insights into the overall health of the hydraulic system, they do not directly confirm whether the filter is performing as intended. Similarly, a stable system gauge may reflect consistent system performance but does not specifically address the filter's condition. Thus, the status of the filter bypassed light offers the clearest indication of proper filter operation.