How should you test the tractor-semi trailer connection for security?

To ensure the security of the tractor-semi trailer connection, the most reliable method is to pull gently forward in low gear against the locked trailer brakes. This approach allows the driver to apply power to the tractor while the trailer is held stationary by its brakes, which helps to confirm that the connection is secure. If the connection is not secure, the trailer may separate from the tractor when pulling forward, indicating that further inspection or adjustment is required.

In contrast, other methods can be less effective or may not provide the necessary assurance of a secure connection. For instance, a visual inspection alone might not reveal problems such as improper coupling or a damaged locking mechanism. Pulling forward in high gear could risk damaging equipment or creating unsafe conditions, as it may not allow for a controlled test of the connection. Shaking the trailer could give a preliminary check, but it does not provide the same level of confirmation as the method of gently applying power in low gear against the locked brakes. Thus, option C is the best practice for confirming the security of the connection.