Which condition is a potential consequence of wet brakes?

Wet brakes can lead to a significant loss of braking effectiveness because water can reduce the friction between the brake components. When brakes are wet, they may not engage properly, causing longer stopping distances and a delayed response. This loss of control can easily lead to situations such as a trailer jackknife. A jackknife occurs when the trailer swings out from behind the cab, which can happen if the driver cannot stop the vehicle in time or if the trailer brakes do not respond adequately when needed.

In contrast, the other options do not accurately reflect the consequences of wet brakes. Increased fuel efficiency and improved control of the vehicle are benefits associated with proper brake function, not wet conditions. Better brake performance is also compromised with wet brakes, as water interferes with their ability to generate the necessary friction. Thus, the risk of a trailer jackknife is a direct and serious consequence of having wet brakes, making this the correct choice.