Which of the following could indicate a loss of control in a rotary wing aircraft?

A critical vibration in a rotary wing aircraft serves as a significant indicator of a potential loss of control. Vibration can arise from various factors, such as imbalance of the rotor system, structural failures, or mechanical issues in the rotor or drivetrain. These vibrations can lead to pilot difficulties in maintaining control of the aircraft, exacerbating stress on components and potentially leading to a catastrophic failure if not addressed promptly.

In contrast, options like a stable flight path, increased RPM, and sustained lift typically reflect controlled and effective flight conditions. A stable flight path is indicative of good control, while increased RPM may denote an adjustment in engine power or collective control but doesn't inherently signal a loss of control unless associated with a specific condition. Sustained lift is crucial for flight and would generally indicate that the aircraft is operating as intended. Therefore, the presence of critical vibrations is the most concerning sign of a potential loss of control in a rotary wing aircraft.