Answer :
Final answer:
The trailer brakes would be activated by the trailer relay valve in the event of a breakaway. This valve is specifically designed to respond to a loss of air pressure and apply the trailer brakes to prevent a runaway situation.
Explanation:
If the trailer breaks away from the tractor, the trailer brakes would be activated by the action of the trailer relay valve. This is because the relay valve is designed to apply the trailer brakes if there is a loss of air pressure in the brake system, which would occur in the event of a breakaway. The tractor protection valve is involved but specifically acts to protect the air supply on the tractor, while the other options listed (highway valve and check valve) are not directly related to the activation of the trailer brakes during a breakaway scenario.
