Glad hands are typically connected to which of the following?

Glad hands are used to connect the service and emergency air lines between a tractor and a trailer. This connection is critical for the proper functioning of the vehicle's air brake system. The service air line allows air to flow to the trailer’s brakes during normal operation, while the emergency air line provides air pressure to the trailer's emergency brakes in the event of a disconnection or malfunction.

Understanding the purpose and function of glad hands is crucial for drivers, as maintaining a secure connection ensures efficient braking performance and safety while towing a trailer. The other options presented do not involve glad hands; for instance, fuel lines are unrelated to the braking system, electrical systems are connected through different types of junctions or plugs, and while trailer brakes are indeed crucial parts of the trailer operation, their direct connection involves the air lines that glad hands facilitate rather than the brakes themselves.