Answer:
Trailer Hitch & One Brake light
Explanation:
The Trailer Hitch is obvious, it's used to keep the trailer in place so it won't get de-tached from the truck itself, potentially endangering other Motorists, and causing damage.
The Brake Light is required by law so other cars are able to indicate you are attempting to stop (without them, traffic accidents), if a trailer is 30" wide then you only need one.
