Final answer:
During a pre-trip inspection, various components including braking performance should be verified. A freight truck's trailer that bounces up and down slowly on the highway is likely nearly empty, as a loaded trailer would not bounce as much due to its weight.
Step-by-step explanation:
When performing the pre-trip inspection for a truck-tractor or three-axle vehicle, it's crucial to ensure all components are in working order for safety and legal compliance. Though the question presents multiple options, typically during a pre-trip inspection, drivers should verify all of those elements. However, in the context of the reference information given, understanding the braking performance of the vehicle is essential. This includes knowing the distance the driver covers before hitting the brakes, the time it takes for the truck to stop after the brakes are applied, and the total stopping distance. These factors are critical for safe operation on the highway.
When observing a freight truck on the highway, if the trailer is bouncing up and down slowly, it is more likely that the trailer is nearly empty. A heavily loaded trailer will have less bounce due to the weight pressing down on the suspension, thus damping the movement. In contrast, an empty trailer would have more spring in its suspension, causing it to bounce more noticeably with the inconsistencies of the road surface.