Final answer:
The advantage of a sliding fifth wheel is that it allows the adjustment of the weight-over-axle on the tractor, contributing to better stability, legal compliance, and fuel efficiency.
Step-by-step explanation:
One advantage of a sliding fifth wheel is that it allows for the alteration of weight distribution over the axles on a tractor-trailer. Specifically, it enables drivers to change the weight-over-axle on the tractor, which is important for maintaining vehicle stability, complying with road weight regulations, and ensuring efficient fuel consumption.
Adjusting the position of the sliding fifth wheel changes the point at which the load is balanced between the tractor's rear axle(s) and the trailer's axle(s), affecting how weight is distributed on the vehicle. The correct option from the listed choices is: d) Weight-over-axle on the tractor can be altered.