What indicates correct coupling after a trailer has been hooked?

The correct answer reflects the principle that, once a trailer is properly coupled to a towing vehicle, the trailer should be securely attached and can only be moved as a unit with the towing vehicle. This is crucial for safe driving, as it ensures that the trailer is not independently movable and that the connection is stable enough to handle the stresses of driving, turning, and braking.

When the trailer can only be moved by the towing vehicle, it indicates that the coupling mechanism, such as the fifth wheel, has engaged correctly, creating a solid link between the truck and the trailer. This prevents any accidental disconnection or separation while the vehicle is in motion.

For context, other options do not adequately signify correct coupling. If the trailer remains perfectly still without motion, it does not alone confirm a secure coupling; it could also indicate the trailer is chocked or immovable for another reason. Visible space between the fifth wheels clearly indicates an improper connection, as there should be a flush fit without gaps for effective coupling. Finally, while proper brake engagement is essential for safety, it does not directly indicate that the trailer is securely coupled; it merely points to functionality in braking systems.