Which of the following is NOT a component of the hydraulic system?

The correct choice identifies a component that is not part of a hydraulic system. In an aircraft, hydraulic systems are primarily used to operate various mechanical components that require significant force. Common components that utilize hydraulic systems include the speed brake, landing gear, and main gear doors, as these elements need hydraulic power for movement and operation.

Navigation controls, however, typically refer to systems that manage the directional orientation and movements of the aircraft, such as the autopilot and electronic navigation aids. These systems predominantly operate through electronic signals and inputs rather than hydraulic power. Therefore, navigation controls do not belong to the category of components generally associated with hydraulic systems. This distinction clarifies why this choice is the correct answer.