Final answer:
The correct practice when preparing to back up under a semitrailer is to line up directly in front of the trailer, ensuring that the kingpin will properly connect to the fifth wheel without misalignment.
Step-by-step explanation:
When preparing to back a truck under a semitrailer, the most common practice is to line up directly in front of the trailer. This ensures that the truck is aligned with the trailer and that the kingpin can properly connect to the fifth wheel of the truck. It's essential to approach the trailer straight on to engage the locking jaws evenly and avoid any potential for misalignment or damage to the coupling components. It is not typical to specifically aim to engage one locking jaw first, nor is it standard to line up a specific tire with the kingpin, as these practices may not account for the precise alignment needed for a safe and secure coupling.