Which of the following systems is powered by the hydraulic system?

The landing gear system is powered by the hydraulic system because it requires significant force to operate effectively. Hydraulic systems utilize fluid under pressure to perform work, making them ideal for moving heavy components like landing gear up and down during aircraft operations. The landing gear needs to lock into place securely when the aircraft is on the ground and retract smoothly during takeoff and landing, which is efficiently achieved through hydraulic power.

In contrast, communications systems, navigation systems, and electrical systems primarily rely on electrical or electronic components for operation, rather than hydraulic power. While these systems may have some minor hydraulic elements, such as certain actuators, their overall functionality does not depend on hydraulic systems like the landing gear does. Thus, the landing gear clearly stands out as the correct choice in this context.