Which of the following is NOT an effect of a boat and trailer on a vehicle?

Choosing "improved fuel efficiency" as the answer reflects an understanding of the impact that towing a boat and trailer has on a vehicle's performance. When a vehicle tows a heavy load, such as a boat and trailer, the total weight increases significantly. This added weight often leads to decreased fuel efficiency because the engine has to work harder to move the vehicle and the trailer. The additional drag and weight require more power and fuel consumption, which is why improved fuel efficiency is not an expected effect of towing a boat and trailer.

Additionally, towing can lead to other effects like additional wear on brakes due to the increased weight and the need for greater stopping power. It can also affect the vehicle's suspension since the weight of the trailer puts pressure on the hitch and rear suspension system, altering its dynamics. The ability to pull and stop is certainly a necessary feature of a towing vehicle, but it is also affected negatively by the added weight and strain of the trailer. Therefore, improved fuel efficiency stands out as the correct answer, as it defies the common expectations of towing impacts.