Which of the following should be ensured about the trailer when it is being coupled?

When coupling a trailer to a vehicle, ensuring there is enough clearance is crucial for a successful connection. Adequate clearance prevents the trailer from coming into contact with the towing vehicle or any other obstacles during the coupling process. This helps avoid damage to both the trailer and the towing vehicle and ensures safety while maneuvering the combination.

In the context of coupling, clearance refers to the space required for the rear of the towing vehicle and the front of the trailer to come together without interference. If there isn't enough clearance, it could lead to misalignment or damage, making it difficult to properly attach the trailer.

Others aspects, like making sure the trailer is clean, that the weight is balanced, and checking if the lights are functioning, are also important considerations for overall safety and operation but do not directly relate to the physical act of coupling the trailer. Cleanliness can affect visibility and perceptions during inspection, balanced weight assists in safe travel, and functioning lights are necessary for signaling, but these elements do not impact the immediate coupling process.