Why is it critical to lock trailer brakes before backing up?

Answer: B — To ensure the trailer remains stationary

Locking the trailer brakes before backing up is essential primarily to ensure that the trailer remains stationary. When a vehicle is in motion, particularly during the complex task of backing up, there is a risk that the trailer could shift or roll. If the trailer moves unintentionally, it can create dangerous situations, such as misalignment with the towing vehicle or even accidents. By locking the brakes, it stabilizes the trailer, allowing the operator to focus on the backing maneuver without the concern of the trailer shifting unexpectedly. This stability is particularly crucial when aligning the trailer with a loading dock or connecting it to another vehicle. The other options highlight various aspects of backing and maneuvering with a trailer, but the main priority is the prevention of unwanted movement during the process, which the correct answer directly addresses.