A major leak in the trailer emergency line will cause `_________`.

A An air leak could cause all of these things.

explanation

A major leak in the emergency line will cause the tractor protection valve to close and the trailer emergency brakes to come on.
You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.