How can a driver check if the trailer brake lights function correctly?

Answer: B — By having another person activate the brakes

To determine if the trailer brake lights function correctly, the most effective method is to have another person activate the brakes while the driver observes the lights. This approach allows for direct confirmation of whether the brake lights illuminate when the brakes are applied. The alternation between the human element and the light observation also ensures that any issues can be immediately identified, such as bulbs that are burnt out or wiring problems that might not be detectable from the driver's seat. Consulting the vehicle manual, while useful for understanding the trailer's systems, does not provide real-time feedback on the functioning of the brake lights. Checking the fuses is a good troubleshooting step if there is an issue, but it does not immediately indicate whether the lights are operational during the pre-trip inspection. Using a flashlight can help to visually inspect the bulbs but does not confirm their function when the brake pedal is pressed. Therefore, having another person activate the brakes is the most straightforward and effective method for ensuring the trailer brake lights are working as needed.