What is the purpose of checking the coupling device on a tractor-trailer?

The purpose of checking the coupling device on a tractor-trailer is to verify that it is locked securely and functioning properly. This is crucial for safety as the coupling device is what connects the trailer to the tractor, and any failure in this connection can lead to serious accidents, including the trailer detaching while on the road.

During a pre-trip inspection, you would examine the coupling device to ensure that it is properly engaged, free from damage, and that all components, such as the locking mechanism, are working as intended. A secure coupling prevents the trailer from swaying or disconnecting during transit, which is essential for safe operation.

The other options provided do not relate directly to the immediate function and importance of the coupling device. Tire alignment, paint condition, and weight capacity are important aspects of vehicle safety and operation, but they are separate checks that do not involve the verification of the coupling device's security and functionality.