What indicates that a heavy trailer is overloaded?

B is the correct answer because overloaded heavy trailers affect the vehicle's handling and braking. When a trailer is carrying more weight than it’s designed for, the additional load can lead to difficulty in steering due to the increased weight and inertia. This extra load also negatively impacts braking performance, as the brakes must work harder to stop the heavier load, which could result in longer stopping distances or brake failure.

The other options do not describe the conditions associated with an overloaded trailer. Improved braking performance and increased fuel efficiency would be contrary to the effects of an overload, as those would typically diminish with excess weight. Greater stability on the road also contradicts the effects of being overloaded since the added weight can lead to swaying or difficulty controlling the trailer.