Which system serves as the pressure source for power brakes in aircraft?

The correct answer is that the auxiliary hydraulic system serves as the pressure source for power brakes in aircraft. Power brakes require a reliable hydraulic pressure source to function effectively, providing the necessary force to operate the brake components.

In many aircraft, the main hydraulic system is primarily tasked with powering the flight control surfaces and other critical systems, while the auxiliary hydraulic system is specifically designed to support non-essential systems such as the brakes. This division of responsibility allows for redundancy and ensures that critical operations can continue even if the main system experiences an issue.

The emergency brake system typically operates independently of the auxiliary or main hydraulic systems, often using a mechanical or backup hydraulic source. The pneumatic braking system, on the other hand, relies on air pressure and is not generally associated with the hydraulic mechanisms prevalent in most modern aircraft braking systems.

Thus, the auxiliary hydraulic system is specifically tailored to provide the necessary pressure to ensure efficient and responsive braking control, making it the correct choice for this question.