What safety feature is integral to the operation of a trailer's brakes?

Answer: B — Glad hands

Glad hands are crucial for the operation of a trailer's brakes because they are the coupling mechanism that connects the air supply lines between a truck and a trailer. When a trailer is equipped with air brakes, glad hands are used to deliver compressed air from the tractor to the trailer's braking system. This allows the trailer to effectively utilize air pressure to apply its brakes when the driver of the truck activates them. In the context of safely operating a trailer, having properly connected and functioning glad hands ensures that the brakes can engage when needed. This is essential for stopping power and overall safety while towing a trailer. The absence or malfunction of glad hands could lead to brake failure, creating a hazardous situation on the road. Other factors, such as tongue weight, tire pressure monitoring, and transmission fluid level, do play a role in vehicle operation and safety, but they do not directly influence the brake system of a trailer in the same manner as glad hands.