Which devices are operated by the primary hydraulic system?

The primary hydraulic system in the T-6 aircraft is primarily responsible for operating essential components that are critical for flight operations and safe landings. The correct answer identifies the components that are directly controlled by this hydraulic system.

The landing gear is crucial for taking off and landing, providing the necessary support on the runway. The main gear inboard doors manage the aerodynamic efficiency during flight and ensure that the gear is fully stowed when not in use. The flaps assist in increasing lift at lower speeds, allowing for safer landings, and the speed brake enhances control and helps reduce speed during descent and approach.

Understanding this configuration is important because it reflects the dependence of fundamental flight maneuvers and landing safely on the efficient operation of hydraulic systems. The other options include either components that are not primarily controlled by the primary hydraulic system or mention systems associated with different operational scenarios or systems, which makes them incorrect in this context.