Which system could lose brake fluid when connecting a trailer?

The hydraulic system is the correct answer because it relies on fluid to transfer force from the brake pedal to the brake components at the wheels. When connecting a trailer, there is a possibility that the hydraulic lines could be affected, leading to a loss of brake fluid. This could happen if there is a leak in the connection or if the brake lines are compromised in any way during the trailer hookup process.

In contrast, air brake systems use compressed air rather than fluid to operate. A mechanical brake system relies on levers and linkages instead of fluid or air pressure, thus wouldn't normally experience fluid loss. Disc brakes can be part of different systems, including hydraulic, but it’s the overall hydraulic system that is specifically associated with fluid loss issues when connecting trailers.