If you see the IFR probe out while the jet is on the taxiway at night, what is the likely cause?

The presence of an IFR (Instrument Flight Rules) probe being extended while the aircraft is on the taxiway at night is typically an indication that the pilots' taxi light is not functioning properly. The taxi light illuminates the area in front of the aircraft and is used during ground maneuvers to provide visibility for taxiing. If the taxi light is out, the pilots might rely on the IFR probe to provide some level of guidance and situational awareness for navigating the taxiway in low-visibility conditions.

The other choices do not accurately address the specific situation with the IFR probe being out during taxiing. Malfunctioning external lights could affect visibility, but they wouldn't specifically explain the IFR probe's status. Routine maintenance could be a valid consideration, but it doesn't directly connect to the IFR probe being out at night. A failing hydraulic system can impact various aircraft systems, but the specific observation of the IFR probe suggests a more immediate issue related to the visibility and operational status of the taxi light. Thus, the most likely cause ties back to the pilots’ taxi light being out.