What indicates that the trailer is at the right height when coupling a semi trailer to a tractor?

When coupling a semi-trailer to a tractor, the correct indication that the trailer is at the right height is that it will be raised slightly when the tractor is backed under it. This slight raise occurs because the fifth wheel on the tractor is designed to connect with the upper part of the kingpin on the trailer.

As the tractor approaches and backs under the trailer, the curvature and design of the tractor's fifth wheel mechanism ensure that the height is properly aligned, allowing the trailer to lift a bit as the coupling occurs. If the trailer is at the optimal height, this action ensures that the kingpin is engaged properly within the fifth wheel, securing a safe connection.

Other choices do not accurately reflect the proper coupling process. While a level trailer or one visibly lower than the tractor might suggest the trailer is positioned incorrectly, and coupling prior to backing in negates the necessity of aligning the heights correctly, the essence is that the trailer must be slightly lifted during the coupling for a successful engagement.